Add extra warning when "hg push" yields "no changes found" only due to lack of bookmark

Created originally on Bitbucket by Robert Tupelo-Schneck

Sometimes I forget that I’m using hg-git and bookmarks, and I do “hg up tip” or some such and lose the active bookmark. Then when I commit my new commits aren’t under a bookmark, and hg push just says “no changes found”.

It would be very convenient if “hg push” gave some extra warning in this case: that there are changes that would be pushed as far as hg is concerned, except that there’s no bookmark and so hg-git can’t push them.