ghpr: Only label the branch as merged

Stop trying to push to the pull request branches. Only I have permission
to do that. Add 'meged' tag. This isn't quite as good as having the PRs
show up as 'merged' since there's some stats based on that which won't
happen. However, it's too much of a hassle, so just stop doing it.

We now require the 'gh' but don't make sure that it's around. Note: We
still need better error handling, for this and other things.

Sponsored by: Netflix