Fix broken view when using a tag to display a tree that contains git submodules

Handle case where git revision is a tag. Also check that the entry is a valid type (commit vs. tag).

You can reproduce the Error 500 here:

https://gitlab.com/gitlab-org/gitlab-git-test/tree/v1.2.1

Closes https://github.com/gitlabhq/gitlabhq/issues/8411