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)

routers/web/repo/commit.go

@@ -430,7 +430,7 @@ func Diff(ctx *context.Context) {
 func RawDiff(ctx *context.Context) {
 	var gitRepo *git.Repository
 	if ctx.Data["PageIsWiki"] != nil {
-		wikiRepo, err := git.OpenRepository(ctx.Repo.Repository.WikiStorageRepo())
+		wikiRepo, err := git.OpenRepository(ctx, ctx.Repo.Repository.WikiStorageRepo())
 		if err != nil {
 			ctx.ServerError("OpenRepository", err)
 			return