git: Amend (#28187)

Adds git amend support.

- [x] Turn existing commit button into split button
- [x] Clean up + Handle shortcuts/focus cases
- [x] Test remote

Release Notes:

- Added git amend support.

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/git/src/git.rs

@@ -50,6 +50,8 @@ actions!(
         Pull,
         Fetch,
         Commit,
+        Amend,
+        Cancel,
         ExpandCommitEditor,
         GenerateCommitMessage,
         Init,