This PR reworks the `FakeGitRepository` type that we use for testing git
interactions, to make it more realistic. In particular, the `status`
method now derives the Git status from the differences between HEAD, the
index, and the working copy. This way, if you modify a file in the
`FakeFs`, the Git repository's `status` method will reflect that
modification.
Release Notes:
- N/A
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Co-authored-by: Junkui Zhang <364772080@qq.com>
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