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)

cmd/admin.go

@@ -128,7 +128,7 @@ func runRepoSyncReleases(ctx context.Context, _ *cli.Command) error {
 		log.Trace("Processing next %d repos of %d", len(repos), count)
 		for _, repo := range repos {
 			log.Trace("Synchronizing repo %s", repo.FullName())
-			gitRepo, err := git.OpenRepository(repo)
+			gitRepo, err := git.OpenRepository(ctx, repo)
 			if err != nil {
 				log.Warn("OpenRepository: %v", err)
 				continue