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/repository/files/patch.go

@@ -211,7 +211,7 @@ func ApplyDiffPatch(ctx context.Context, repo *repo_model.Repository, doer *user
 		return nil, err
 	}
 
-	fileCommitResponse, _ := GetFileCommitResponse(repo, commit) // ok if fails, then will be nil
+	fileCommitResponse, _ := GetFileCommitResponse(repo, gitRepo, commit) // ok if fails, then will be nil
 	verification := GetPayloadCommitVerification(ctx, commit)
 	fileResponse := &structs.FileResponse{
 		Commit:       fileCommitResponse,