Warn if some changes are not staged for commit before sending

[philmd]

Feature request

I found myself sending series with uncommitted changes few times already. As I believe it happened to others, maybe displaying a dumb warning would be useful to all.

This happened when I wrote the patches, then while writing the cover in git-publish I figure out a better way, so I rework the patches, then git-publish again to modify the cover message but forgot to commit the unstaged changes...

Anyway this is a selfish feature request, so I might implement it one day and add an issue here to not forget about it. Any quicker volunteer welcome to beat me at it :)