Fold git merge messages into commit editor placeholder text (#26992)

This PR changes the git commit message editors to surface git's
suggested merge message, if any, as placeholder text, as opposed to
"real" buffer text as was previously the case.

Release Notes:

- Changed git commit message editors to use placeholder text for git's
suggested merge messages

crates/git_ui/src/commit_modal.rs

@@ -136,7 +136,7 @@ impl CommitModal {
         cx: &mut Context<Self>,
     ) -> Self {
         let panel = git_panel.read(cx);
-        let suggested_commit_message = panel.suggest_commit_message();
+        let suggested_commit_message = panel.suggest_commit_message(cx);
 
         let commit_editor = git_panel.update(cx, |git_panel, cx| {
             git_panel.set_modal_open(true, cx);