Branches not at ref commit ID should not be listed as Merged (#9614)

Once a branch has been merged if the commit ID no longer equals that of
the pulls ref commit id don't offer to delete the branch on the pull screen
and don't list it as merged on branches.

Fix #9201

When looking at the pull page we should also get the commits from the refs/pulls/x/head

Fix #9158

models/pull.go

@@ -122,7 +122,7 @@ func (pr *PullRequest) LoadHeadRepo() error {
 		if has, err := x.ID(pr.HeadRepoID).Get(&repo); err != nil {
 			return err
 		} else if !has {
-			return ErrRepoNotExist{ID: pr.BaseRepoID}
+			return ErrRepoNotExist{ID: pr.HeadRepoID}
 		}
 		pr.HeadRepo = &repo
 	}