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/repository/branch.go

@@ -225,7 +225,7 @@ func loadOneBranch(ctx context.Context, repo *repo_model.Repository, dbBranch *g
 		if pr.HasMerged {
 			baseGitRepo, ok := repoIDToGitRepo[pr.BaseRepoID]
 			if !ok {
-				baseGitRepo, err = git.OpenRepository(pr.BaseRepo)
+				baseGitRepo, err = git.OpenRepository(ctx, pr.BaseRepo)
 				if err != nil {
 					return nil, fmt.Errorf("OpenRepository: %v", err)
 				}