Reduce amount of monomorphizations from FnMut closures (#49453)

Replaces a bunch of `impl FnMut` parameters with `&mut dyn FnMut` for
functions where this is the sole generic parameter.
Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/git_ui/src/commit_view.rs

@@ -958,7 +958,7 @@ impl Item for CommitView {
         }))))
     }
 
-    fn to_item_events(event: &EditorEvent, f: impl FnMut(ItemEvent)) {
+    fn to_item_events(event: &EditorEvent, f: &mut dyn FnMut(ItemEvent)) {
         Editor::to_item_events(event, f)
     }