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/merge_squash.go

@@ -25,7 +25,7 @@ func getAuthorSignatureSquash(ctx *mergeContext) (*git.Signature, error) {
 	// Try to get a signature from the same user in one of the commits, as the
 	// poster email might be private or commits might have a different signature
 	// than the primary email address of the poster.
-	gitRepo, err := git.OpenRepositoryLocal(ctx.tmpBasePath)
+	gitRepo, err := git.OpenRepositoryLocal(ctx, ctx.tmpBasePath)
 	if err != nil {
 		log.Error("%-v Unable to open base repository: %v", ctx.pr, err)
 		return nil, err