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)

modules/util/runtime_test.go

@@ -12,7 +12,7 @@ import (
 
 func TestCallerFuncName(t *testing.T) {
 	s := CallerFuncName()
-	assert.Equal(t, "gitea.dev/modules/util.TestCallerFuncName", s)
+	assert.Equal(t, "util.TestCallerFuncName", s)
 }
 
 func BenchmarkCallerFuncName(b *testing.B) {