refactor: decouple git.Repository(ctx) from git.Commit & git.Tree (#38464)

1. Storing "ctx" in a long-living object is wrong
2. Make the commit & tree cacheable (for the future performance
optimization)
3. Also fix some bad designs like `// FIXME: bad design, this field can
be nil if the commit is from "last commit cache"`

ref:
* #33893

services/actions/scoped_workflow_cache.go

@@ -60,7 +60,7 @@ func LoadParsedScopedWorkflows(ctx context.Context, sourceRepo *repo_model.Repos
 	if err != nil {
 		return "", nil, fmt.Errorf("get source commit %s: %w", sha, err)
 	}
-	parsed, err = actions_module.ParseScopedWorkflows(sourceCommit)
+	parsed, err = actions_module.ParseScopedWorkflows(ctx, sourceGitRepo, sourceCommit)
 	if err != nil {
 		return "", nil, err
 	}