Tests paths from a subdirectory of the Git repo

[jdblischak]

I added some tests related to the observations in Issue #394.

Currently the behavior of functions like add(), rm_file(), and commits(path) change based on the operating system, the current working directory, whether or not the file exists, and the type of file path (relative versus absolute).

As an example, when the file exists, the commands commits(repo, path = "../root.txt") and commits(repo, path = "root.txt") both return the commits that touch the file root.txt in the root of the Git repo. After the file has been removed, commits(repo, path = "../root.txt") returns an empty list, but commits(repo, path = "root.txt") still works.

> file.exists("../root.txt")
[1] TRUE
> commits(repo, path = "../root.txt")
[[1]]
[a7f30ea] 2019-11-07: Add files

> commits(repo, path = "root.txt")
[[1]]
[a7f30ea] 2019-11-07: Add files

> ## Remove and commit
> rm_file(repo, c(root, sub))
> commit(repo, "Remove files")
[1d77e3b] 2019-11-07: Remove files
> # after removing the file
> file.exists("../root.txt")
[1] FALSE
> commits(repo, path = "../root.txt")
list()
> commits(repo, path = "root.txt")
[[1]]
[1d77e3b] 2019-11-07: Remove files

[[2]]
[a7f30ea] 2019-11-07: Add files

The crux of the problem is that relative paths are interpreted in 2 distinct ways: relative to the current working directory or relative to the root of the Git repository.