When checkout is a hash, some messages could be better

[MurphyMc]

@AbePralle has a gist which shows some unhelpful messages.

These all revolve around a case where the repo's checkout is a specific commit.

• For example on line line 14/16, it looks like the advice here is slightly wrong. It suggests checking out the commit directly, but depz should have created a branch for that commit. I think you should probably check that out.
• Bonus: We should probably have a sanity check where if the current branch is checkout_<hash>, the HEAD should actually be the given hash!
• On line 34/39, it's doing do_show_outdated and complains that it can't tell. There's a special case if the checkout is a tag, since a tag can't be outdated. I think we should attempt to apply similar logic when the checkout is a specific commit. If there are monitored branches, we should check them, but if there aren't, we should just assume things are okay (as long as the checkout commit is in the ancestors of HEAD).
• On line 35/40... this one is debatable. The warning might be fine, and the repo should just be configured with fast_forward_skip. I think that's probably best. (One could argue that if the checkout is a hash, we should just skip it, but for a couple reasons, I don't like this as well.)