git: on import_refs(), don't clobber view's heads with known HEAD@git

In colocated mid-size "linux" repo, this saves ~450ms needed to do
enforce_view_invariants(). We could instead make add_head() to return early,
but the condition would be a bit weird since HEAD@git is typically a parent
of known heads, not a head itself.

lib/src/git.rs

@@ -130,11 +130,13 @@ pub fn import_some_refs(
         // to `old_git_heads` because HEAD move doesn't automatically mean the old
         // HEAD branch has been rewritten.
         let head_commit_id = CommitId::from_bytes(head_git_commit.id().as_bytes());
-        let head_commit = store.get_commit(&head_commit_id).unwrap();
         new_git_heads.insert("HEAD".to_string(), vec![head_commit_id.clone()]);
-        prevent_gc(git_repo, &head_commit_id)?;
-        mut_repo.add_head(&head_commit);
-        mut_repo.set_git_head(RefTarget::Normal(head_commit_id));
+        if !matches!(mut_repo.git_head(), Some(RefTarget::Normal(id)) if id == head_commit_id) {
+            let head_commit = store.get_commit(&head_commit_id).unwrap();
+            prevent_gc(git_repo, &head_commit_id)?;
+            mut_repo.add_head(&head_commit);
+            mut_repo.set_git_head(RefTarget::Normal(head_commit_id));
+        }
     } else {
         mut_repo.clear_git_head();
     }

lib/src/repo.rs

@@ -947,6 +947,10 @@ impl MutableRepo {
         self.view_mut().remove_git_ref(name);
     }
 
+    pub fn git_head(&self) -> Option<RefTarget> {
+        self.view.with_ref(|v| v.git_head().cloned())
+    }
+
     pub fn set_git_head(&mut self, target: RefTarget) {
         self.view_mut().set_git_head(target);
     }