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)

modules/git/languagestats/language_stats_get.go

@@ -21,7 +21,7 @@ import (
 func GetLanguageStats(ctx context.Context, repo *git.Repository, commitID string) (map[string]int64, error) {
 	// We will feed the commit IDs in order into cat-file --batch, followed by blobs as necessary.
 	// so let's create a batch stdin and stdout
-	batch, cancel, err := repo.CatFileBatch(ctx)
+	batch, cancel, err := repo.CatFileBatch()
 	if err != nil {
 		return nil, err
 	}