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)

tests/integration/api_repo_file_update_test.go

@@ -136,7 +136,7 @@ func TestAPIUpdateFile(t *testing.T) {
 			req := NewRequestWithJSON(t, "PUT", fmt.Sprintf("/api/v1/repos/%s/%s/contents/%s", user2.Name, repo1.Name, treePath), &updateFileOptions).
 				AddTokenAuth(token2)
 			resp := MakeRequest(t, req, http.StatusOK)
-			gitRepo, _ := git.OpenRepository(repo1)
+			gitRepo, _ := git.OpenRepository(t.Context(), repo1)
 			defer gitRepo.Close()
 			commitID, _ := gitRepo.GetBranchCommitID(t.Context(), updateFileOptions.NewBranchName)
 			lasCommit, _ := gitRepo.GetCommitByPath(t.Context(), treePath)