Reuse existing logic used to generate commit messages to disable commit buttons (#26034)

Also
- Recomputes `suggested_commit_message` and no longer stores it, to
ensure things are always up to date
- Reduces indentation in `render_footer`

Release Notes:

- N/A

crates/git_ui/src/commit_modal.rs

@@ -337,6 +337,7 @@ impl CommitModal {
     fn dismiss(&mut self, _: &menu::Cancel, _: &mut Window, cx: &mut Context<Self>) {
         cx.emit(DismissEvent);
     }
+
     fn commit(&mut self, _: &git::Commit, window: &mut Window, cx: &mut Context<Self>) {
         self.git_panel
             .update(cx, |git_panel, cx| git_panel.commit_changes(window, cx));