Add a --dry-run option to git-push.

Add a --dry-run option to git-push.

The default behaviour of git-push is potentially confusing
for new users, since it will push changes that are not on
the current branch. Publishing patches that were still
cooking on a development branch is hard to undo.

It would also be nice to be able to verify the expansion
of refspecs if you've edited them, so that you know
what branches matched on the server.

Adding a --dry-run flag allows the user to experiment
safely and learn how to use git-push properly. Originally
suggested by Steffen Prohaska.

Signed-off-by: Brian Ewins <brian.ewins@gmail.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>