chore: still keep ctx in git.Repository struct for cat-file batch command (#38684)

Unfortunately, we can't completely remove the ctx from git.Repository,
because the CatFileBatch still heavily depends on a parent context.

If we remove the Repository ctx, the CatFileBatch will become a mess and
create a lot of unnecessary git processes.

http://localhost:3000/-/admin/monitor/perftrace

* Before: open a repo home, dozens of git processes (duplicate cat-file)
* After: only a few (no duplicate cat-file)

services/pull/comment_test.go

@@ -25,7 +25,7 @@ func TestCreatePushPullCommentForcePushDeletesOldComments(t *testing.T) {
 	require.NoError(t, pr.LoadIssue(ctx))
 	require.NoError(t, pr.LoadBaseRepo(ctx))
 
-	gitRepo, err := git.OpenRepository(pr.BaseRepo)
+	gitRepo, err := git.OpenRepository(ctx, pr.BaseRepo)
 	require.NoError(t, err)
 	defer gitRepo.Close()