Merge branch 'en/update-commit-map-and-ref-map'

Instead of overwriting the $GIT_DIR/filter-repo/{commit,ref}-map files
after every run, if someone does filtering in multiple invocations of
git-filter-repo then it is more useful to update these files instead of
overwrite.  Add new datastructures and modify the flow to enable and do
this.

Signed-off-by: Elijah Newren <[email protected]>

Documentation/git-filter-repo.txt

@@ -377,7 +377,7 @@ OUTPUT
 ------
 
 Every time filter-repo is run, files are created in the `.git/filter-repo/`
-directory. These files are overwritten unconditionally on every run.
+directory. These files are updated or overwritten on every run.
 
 Commit map
 ~~~~~~~~~~
@@ -421,10 +421,18 @@ Concretely, this means:
     and support usecase.  You already passed or bypassed the "Fresh Clone"
     check on your initial run.
 
+  * The commit-map and ref-map files above will be updated rather than
+    simply rewritten.
+
+    In other words, if the first filter-repo invocation rewrote commit
+    A to commit B, and the second filter-repo invocation rewrite
+    commit B to commit C, then the second run would have an "A C"
+    entry rather than a "B C" entry for the changed commit.
+
 However, if the already_ran file exists but is older than 1 day when they
 invoke git-filter-repo, the user will be prompted for whether the new run
 should be considered a continuation of the previous run.  If they do not
-answer in the affirmative, then the above bullet will not apply.
+answer in the affirmative, then the above two bullets will not apply.
 This prompt exists because users might do a history rewrite in a repository,
 forget about it and leave the $GIT_DIR/filter-repo directory around, and
 then some months or years later need to do another rewrite.  If commits