[BZ#699689] Push dialog should show what will be pushed

Submitted by Adam Dingle

Description

I know that I was the one who asked to be able to 'git push' from Meld. But now that that's implemented, it's making me a little uncomfortable that there's a button on the Meld toolbar I can push (no pun intended) which will push commits to the server without asking for confirmation. That's a potentially dangerous operation if I hang around in the master branch with local commits which are not yet ready for prime time (as I sometimes do, for better or for worse).

So now I think I'd like that button to ask for confirmation before pushing. Of course, it would be even nicer if the confirmation dialog showed me the first lines of the log messages for the commits that will be pushed. But if that's hard, even a simple Push/Cancel dialog would be a lot better than nothing.