git: Implement commit creation (#23263)

- [x] Basic implementation
- [x] Disable commit buttons when committing is not possible (empty
message, no changes)
- [x] Upgrade GitSummary to efficiently figure out whether there are any
staged changes
- [x] Make CommitAll work
- [x] Surface errors with toasts
  - [x] Channel shutdown
  - [x] Empty commit message or no changes
  - [x] Failed git operations
- [x] Fix added files no longer appearing correctly in the project panel
(GitSummary breakage)
- [x] Fix handling of commit message

Release Notes:

- N/A

---------

Co-authored-by: Nate <nate@zed.dev>

crates/worktree/src/worktree.rs

@@ -231,6 +231,10 @@ impl RepositoryEntry {
         self.statuses_by_path.summary().item_summary.count
     }
 
+    pub fn status_summary(&self) -> GitSummary {
+        self.statuses_by_path.summary().item_summary
+    }
+
     pub fn status_for_path(&self, path: &RepoPath) -> Option<StatusEntry> {
         self.statuses_by_path
             .get(&PathKey(path.0.clone()), &())