Remove git2 (libgit2) dependency (#53453)

## Summary

- Remove the `git2` crate and its C dependencies (`libgit2-sys`,
`libz-sys`), replacing all remaining usage with the git CLI and other
previously used rust crates
- Replace `git2::Patch` in `buffer_diff` with `imara-diff` (used
elsewhere in the codebase for word diffing)
- Replace `git2::Oid` with `[u8; 20]` and the `hex` crate
- Replace `git2::Repository` in `RealGitRepository` with stored paths
and git CLI calls via the existing `GitBinary`
- Fixes a bug where linked worktree git dir events could cause the
repository entry to be dropped

### Motivation

libgit2 does not support the
[reftable](https://github.blog/2024-04-29-highlights-from-git-2-45/#preliminary-reftable-support)
storage format that was introduced in git 2.45 (see
[libgit2#7117](https://github.com/libgit2/libgit2/pull/7117)), meaning
that git operations in the app using these (like git status, branch
names, diffs, e.g.) appeared broken for any repository using
`--ref-format=reftable`. [Git 3 will use reftables by
default](https://www.deployhq.com/blog/git-3-0-on-the-horizon-what-git-users-need-to-know-about-the-next-major-release),
so this needs to be supported eventually.

Most operations used the git binary already, but there were still a
handfull of stragglers using libgit2. By swapping out the remaining
uses, we can remove the dependancy of libgit2 and eliminate ~30k lines
of vendored C and the associated build complexity (cmake, libz,
pkg-config), as well as ensure that all git operations go through
similar codepaths.

Closes #46747
Closes https://github.com/zed-industries/zed/discussions/45702

fixes ZED-76X
fixes ZED-73N

### Notes

- `reload_index` is removed from the `GitRepository` trait since both
implementations were no-ops (the CLI always reads from disk)
- `change_branch` is simplified to `git checkout <name>`, which natively
handles local/remote branch resolution
- `load_index_text` and `load_committed_text` no longer explicitly
filter symlinks (the old code returned `None` for symlinked entries)

## Test plan

- [x] `cargo test -p git` (34 tests)
- [x] `cargo test -p buffer_diff` (14 tests)
- [x] `cargo test -p worktree --
test_linked_worktree_git_dir_events_do_not_panic`
- [x] `cargo test -p util` (108 tests)
- [x] `cargo test -p project -- test_file_status
test_repository_subfolder_git_status test_update_gitignore`
- [ ] Manual testing of diff gutter, staging, branch switching, remote
operations
- [ ] Manual testing with a reftable repository

Release Notes:

- Fixed git integration not working with repositories using the reftable
reference storage format

---------

Co-authored-by: Anthony Eid <anthony@zed.dev>