Do not push to source repository if there was no change

[Jellby]

In the simplest case where the MR can be immediately merged, there's no need for Marge to push to the source repository. In the case of a MR originating from a fork, Marge could have no permission to push, and then the merge fails for no good reason.

Proper handling of MRs from forks will come in a different PR (#113), but this looks like a harmless change that could be accepted right away.

[aschmolck]

I don't think adding extra logic to special case this is a good idea, because now rather than sorting out your permissions you have a situation where sometimes things fail after they appeared to work fine before.

I'd much rather keep it so that if you want to use marge, it needs to be able to push to your branch.

[Jellby]

With the merge strategy there's no need to push to source branches (for reasons to use the merge strategy, see #104 (comment)). Would it be acceptable if the check is done on the merge strategy instead?

[Jellby]

@aschmolck With the merge strategy there's no reason for marge to push to other branches. Would it be acceptable to skip the push when using the merge strategy?

Even with the rebase strategy I don't see a problem with pushing only when necessary. If marge has no permission to push and it's not needed, the merge can proceed anyway. If a push is needed, she will leave a message. Is that worse than leaving a message and failing no matter what, even if no push is needed?