topic: treat argument to --topic as a literal topic name explicitly

We already do treat it as a string (i.e. not a special revset language symbol)
that needs to be escaped, because we use %s in repo.revs(), which escapes and
single-quotes the appropriate argument. But now we abort in case user supplies
topic name that doesn't exist.

Before this patch, when user ran something like `hg push --topic .`, this code
would use 'topic(".") - obsolete()' revset, which would get an empty set from
repo.revs() unless there was a topic named "." (unlikely, because this name is
reserved). This code would then add an empty set to the 'rev' argument of the
original hg push command. And `hg push` without -r will try and push
everything, and not just changesets on a specific topic.