Make FakeGitRepository behave more like a real git repository (#26961)

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

---------

Co-authored-by: Junkui Zhang <364772080@qq.com>

Cargo.lock

@@ -5238,8 +5238,8 @@ dependencies = [
  "fsevent",
  "futures 0.3.31",
  "git",
- "git2",
  "gpui",
+ "ignore",
  "libc",
  "log",
  "notify 6.1.1",