Add item to ideas list.

We've had a few cases of people accidentally adding more commits to a
branch than they wanted; for example, by merging from a branch
sprouted from master into a maintenance branch.  If they had seen that
they were adding many hundreds of commits to the maintenance branch,
they would have hopefully noticed the problem right away and been able
to fix it.  The git-multimail script already knows what commits are
being added by a push; perhaps there should be an option to output
this information to the pusher.