support for lwt 2.5.0, bug fixes and content-type for the REST API

• Allow raw bodies in queries and responses for the REST API. This is
  controlled by the Content-type field set by the client:
  by default, we still use JSON (or use application/json) but using
  application/octet-stream will avoid having to hex-encode large
  binary blobs to make them JSON-compatible. This feature is still
  experimental (especially when using Git on the server) (#255)
• Adapt to ocaml-git.1.7.1 (which works with lwt.2.5.0)
• Expose Store.config for all the stores (AO, RW, etc.)
• Expose Irmin_git.Internals to be able to get back the
  Git commit objects from an head value (#245, #241)
• Expose Irmin.Private.remove_node
• Remove the special __root__ filename in Irmin stores and in views
  (#233)
  • This fixes View.update_path when the view contains a value at its
    root. Now the updated path contains a the value stored at the root
    of the view.
  • Writing a value to the root of a store is now an error
  • Reading a value at the root of a store always return None
• Make the HTTP backend re-raise the Invalid_argument and Failure
  exceptions that were raised by the server.

Mirage support

• Fix wrong interaction of in-memory views and temporary branches in the store
  (#237)
• Fix Irmin.update_tag for HTTP clients
• Initial MirageOS support. Expose Mirage_irmin.KV_RO to surface an
  Irmin store as a read-only key/value store implementing `V1_LWT.KV_RO
  (#107)
• Expose `Irmin_git.Memory_ext. This allows the Git memory backend to be
  configured with a non-empty conduit context.
• Expose Irmin.SYNC
• Transmit client tasks to the HTTP server on DELETE too (#227, @dsheets)
• Do note expose private types in the public interface (#234, @koleini)
• Fix missing zero padding for date pretty-printing (#228, @dsheets)
• Update the tests to use ocaml-git.1.6.0
• Improve the style of the HTTP commit graph.
• Constraint the string tags to contain only alpha-numeric characters
  and few mores (-, _, '.' and /) (#186)
• Fix a race condition in Irmin.clone. (#221)
• Escpate double quotes in the output of commit messages to workaround
  HTML display issues. (#222)

Fix regression in the Git backend, add HTTP API versionning

• Add a version check for HTTP client and server. The client might add the
  version in the HTTP headers using the X-IrminVersion header - the server
  might decide to enfore the version check or not. The server always reply
  with its version in the JSON reply, using a version field. The client
  might use that information to bail out nicely instead of failing because
  of some random unmarshalling errors due to API changes (#167)
• Fix a regression in 0.9.5 and 0.9.6 when inserting new child in Git trees.
  This could cause a tree to have duplicate childs having the same names,
  which would confuse the merge functions, make git fsck and git gc
  complain a lot (with good reasons) and do some fency things with git
  index. The regression has been introduced while trying to fix #190 (the fix
  is in #229)

Fix slice serialisation and race in watch initialisation

• Fix the datamodel: it is not possible to store data in intermediate nodes
  anymore (#209)
• Fix serialization of slices (#204)
• Do not fail silently when the synchronisation fails (#202)
• Fix a race in the HTTP backend between adding a watch and updating the store.
  In some cases, the watch callback wasn't able to see the first few updates
  (#198)
• Fix a race for all the on-disk backends between adding a watch and updating
  the store. This is fixed by making Irmin.Private.Watch.listen_dir and
  Irmin.Private.Watch.set_listen_dir_hook synchronous.
• Update the tests to use alcotest >= 0.4. This removes the dependency towards
  OUnit and nocrypto for the tests.
• Make the file-locking code a bit more robust

New watch API, fixes to export, views, lca computation. Remove Snapshots.

• Fix Irmin.export for the HTTP backend (#196, patch from Alex Zatelepin)
• Fix a race in Irmin.export (#196, patch from Alex Zatelepin)
• Add Task.empty (the empty task) and Task.none (the empty task constructor)
• Completely rewrite the notification mechanism. All the watch functions now
  take a callback as argument and return a de-allocation function. The callbacks
  receive a heads values (the last and current ones) and diff values. (#187)
  • Add Irmin.watch_head to watch for the changes of the current branch's head
  • Add Irmin.watch_tags to watch for the changes of all the tags in the store
  • Add Irmin.watch_key to watch for the changes of the values associated to a
    given key (this is not recursive anymore).
  • Add View.watch_path to watch for the changes in a subtree. The function
    return views and the user can use View.diff to compute differences between
    views if needed.
• Transfer the HTTP client task to the server to make the commit messages
  relative to the client state (and not the server's) (#136)
• Fix View.remove to clean-up empty directories (#190)
• Fix the ordering of tree entries in the Git backend (#190)
• Allow to create a new head from a view and a list of parents with
  View.make_head (#188)
• Allow to create an empty temporary branch with Irmin.empty (#161)
• Use a pure OCaml implementation of SHA1, do not depend on nocrypto anymore
  (#183, by @talex5)
• Remove Irmin.Snapshot. Nobody was using it and it can be easily replaced by
  Irmin.head, Irmin.watch_head and Irmin.update_head.
• Change signature of Irmin.iter to include the values and move it into
  the Irmin.RO signature.
• Add Irmin.fast_forward_head (#172)
• Add Irmin.compare_and_set_head (#171)
• Simplify the RW_MAKER signature (#158)
• Fix Irmin_git.RW_MAKER (#159)
• Improve the efficiency of the LCA computation (#174, with @talex5 help)
• By default, explore the full graph when computing the LCAs. The previous
  behavior was to limit the depth of the exploration to be 256 by default.

better concurrency properties

• Ensure that Irmin.update and Irmin.merge are atomic.
• Fix Irmin.clone of an empty branch
• Add Irmin.RW.compare_and_test that the backends now have to implement
  to guarantee atomicity of Irmin's high-level operations.
• Add Irmin.Private.Lock to provide per-handler, per-key locking. This
  can be used by backend to implement simple locking policies.
• Add Lwt.t to the return type of Irmin.tag and Irmin.tag_exn
• Do not throw [Not_found]. Now all the _exn function raise Invalid_argument
  (#144)
• Remove Irmin.switch and Irmin.detach
• Add Irmin.history to get the branch history as a DAG of heads (#140).
• Fix the computation of lcas (#160)

0.9.3

• Fix the invalidation of the view caches (report by @gregtatcam).
  This was causing some confusing issues where views' sub-keys where
  not properly updated to to their new values when the view is merged
  back to the store. The issues is a regression introduced in 0.9.0.
• Add post-commit hooks for the HTTP server.
• Add Irmin.watch_tags to monitor tag creation and desctructions.
• Fix Irmin.push
• Add Irmin.with_hrw_view to easily use transactions.
• Add a phantom type to Irmin.t to denote the store capabilities
  read-only, read-write or branch-consistent.
• The ~old argument of a merge function can now be optional to
  signify that there is no common ancestor.
• Expose Irmin.with_rw_view to create a temporary, in-memory and
  mutable view of the store. This can be used to perform atomic
  operations in the store (ie. non-persistent transactions).
• Simplify the view API again
• Expose the task of previous commits. This let the user access
  the Git timestamp and other info such as the committer name (#90)
• The user-defined merge functions now takes an unit -> 'a result Lwt.t argument for ~old (instead of 'a). Evalutating the
  function will compute the least-common ancestors. Merge functions
  which ignore the old argument don't have to pay the cost of
  computing the lcas anymore.
• Expose S.lcas to get the least common ancestors
• Update to ocaml-git 1.4.6

0.9.0 bug-fix

• Fix S.of_head for the HTTP client (regression introduced in 0.9.0)
• Fix regression in displaying the store's graph over HTTP introduced by
  0.9.0.
• Fix regression in watch handling introduced in 0.9.0.
• Fix regressions in Views introduced in 0.9.0. (thx @buzzheavyyear for
  the report)
• Always add a commit when calling a update function (Irmin.update
Irmin.remove, Irmin.remove_rec) even if the contents' store have
  not changed.
• The [head] argument of [Git_unix.config] now has a proper type.
• Expose synchronisation functions for basic Irmin stores.
• The user-provided merge function now takes optional values. The
  function is now called much more often during recursive merges
  (even if one of the 3 buckets of the 3-way merge function is not
  filled -- in that case, it uses None).
• Also expose the type of the keys in the type basic Irmin stores. Use
  ('key, 'value) Irmint.t instead of 'value Irmin.t.
• The user-defined merge functions now take the current filename being
  merged as an additional argument.
• The user-defined Contents should expose a Path sub-module. Keys of
  the resulting Irmin store will be of type Path.t.
• Fix irmin init --help. (#103)

Support for Cohttp 0.14.0+ interface

This point release updates the Irmin HTTP layer to be compatible with Cohttp 0.14.0 and higher.

Improved efficiency and removal of Core_kernel dependency

• Improve the efficiency of the Git backend
• Expose a cleaner API for the Unix backends
• Expose a cleaner public API
• Rename Origin into Task and use it pervasively through the API
• Expose a high-level REST API over HTTP (#80)
• Fix the Git backend to stop constantly overwrite .git/HEAD (#76)
• Add a limit on concurrently open files (#93, #75)
• Add remove_rec to remove directories (#74, #85)
• Remove dependency to core_kernel (#22, #81)
• Remove dependency to cryptokit andsha1and usenocrypto` instead
• Remove dependency to caml4
• Fix writing contents at the root of the store (#73)
• More efficient synchronization protocol between Irmin stores (#11)