Finish removing git repository state and scanning logic from worktrees (#27568)

This PR completes the process of moving git repository state storage and
scanning logic from the worktree crate to `project::git_store`.

Release Notes:

- N/A

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Conrad <conrad@zed.dev>

crates/sum_tree/src/tree_map.rs

@@ -317,10 +317,18 @@ where
         ))
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
     pub fn insert(&mut self, key: K) {
         self.0.insert(key, ());
     }
 
+    pub fn remove(&mut self, key: &K) -> bool {
+        self.0.remove(key).is_some()
+    }
+
     pub fn extend(&mut self, iter: impl IntoIterator<Item = K>) {
         self.0.extend(iter.into_iter().map(|key| (key, ())));
     }