Diff view (#32922)

Todo:

* [x] Open diffed files as regular buffers
* [x] Update diff when buffers change
* [x] Show diffed filenames in the tab title
* [x] Investigate why syntax highlighting isn't reliably handled for old
text
* [x] remove unstage/restore buttons

Release Notes:

- Adds `zed --diff A B` to show the diff between the two files

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>
Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/git_ui/Cargo.toml

@@ -57,6 +57,7 @@ time.workspace = true
 time_format.workspace = true
 ui.workspace = true
 util.workspace = true
+watch.workspace = true
 workspace-hack.workspace = true
 workspace.workspace = true
 zed_actions.workspace = true