workspace: Handle non-cloneable items better (#41215)

When trying to split and clone a non clone-able workspace item we now
attempt split and move instead of doing nothing. Additionally we disable
the split menu buttons if we can't split the active item at all.

Release Notes:

- Improved handling of unsplittable panes

crates/git_ui/src/commit_view.rs

@@ -556,6 +556,10 @@ impl Item for CommitView {
         });
     }
 
+    fn can_split(&self) -> bool {
+        true
+    }
+
     fn clone_on_split(
         &self,
         _workspace_id: Option<workspace::WorkspaceId>,