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/gitgraph/graph_test.go

@@ -16,7 +16,7 @@ import (
 )
 
 func BenchmarkGetCommitGraph(b *testing.B) {
-	currentRepo, err := git.OpenRepositoryLocal(".")
+	currentRepo, err := git.OpenRepositoryLocal(b.Context(), ".")
 	if err != nil || currentRepo == nil {
 		b.Error("Could not open repository")
 	}