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>

Cargo.lock

@@ -7620,7 +7620,6 @@ dependencies = [
  "collections",
  "derive_more",
  "futures 0.3.32",
- "git2",
  "gpui",
  "http_client",
  "itertools 0.14.0",
@@ -7647,18 +7646,6 @@ dependencies = [
  "ztracing",
 ]
 
-[[package]]
-name = "git2"
-version = "0.21.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ddddbf932745a6be37109b6112d3ee09696106f848449069d3a57bba937ab82e"
-dependencies = [
- "bitflags 2.10.0",
- "libc",
- "libgit2-sys",
- "log",
-]
-
 [[package]]
 name = "git_graph"
 version = "0.1.0"
@@ -10419,18 +10406,6 @@ dependencies = [
  "cc",
 ]
 
-[[package]]
-name = "libgit2-sys"
-version = "0.18.4+1.9.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9b26f66f35e1871b22efcf7191564123d2a446ca0538cde63c23adfefa9b15b7"
-dependencies = [
- "cc",
- "libc",
- "libz-sys",
- "pkg-config",
-]
-
 [[package]]
 name = "libloading"
 version = "0.8.9"
@@ -10503,18 +10478,6 @@ dependencies = [
  "webrtc-sys",
 ]
 
-[[package]]
-name = "libz-sys"
-version = "1.1.22"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8b70e7a7df205e92a1a4cd9aaae7898dac0aa555503cc0a649494d0d60e7651d"
-dependencies = [
- "cc",
- "libc",
- "pkg-config",
- "vcpkg",
-]
-
 [[package]]
 name = "line_ending_selector"
 version = "0.1.0"

Cargo.toml

@@ -595,7 +595,7 @@ futures = "0.3.32"
 futures-concurrency = "7.7.1"
 futures-lite = "1.13"
 gh-workflow = { git = "https://github.com/zed-industries/gh-workflow", rev = "37f3c0575d379c218a9c455ee67585184e40d43f" }
-git2 = { version = "0.21.0", default-features = false, features = ["vendored-libgit2", "unstable-sha256"] }
+
 globset = "0.4"
 heapless = "0.9.2"
 handlebars = "4.3"

crates/git/Cargo.toml

@@ -20,7 +20,6 @@ askpass.workspace = true
 async-trait.workspace = true
 collections.workspace = true
 derive_more.workspace = true
-git2.workspace = true
 gpui.workspace = true
 http_client.workspace = true
 itertools.workspace = true