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/project_panel/src/project_panel.rs

@@ -1588,7 +1588,7 @@ impl ProjectPanel {
                         }
                     }))
                     && entry.is_file()
-                    && entry.git_summary.modified > 0
+                    && entry.git_summary.index.modified + entry.git_summary.worktree.modified > 0
             },
             cx,
         );
@@ -1666,7 +1666,7 @@ impl ProjectPanel {
                         }
                     }))
                     && entry.is_file()
-                    && entry.git_summary.modified > 0
+                    && entry.git_summary.index.modified + entry.git_summary.worktree.modified > 0
             },
             cx,
         );