Include shortlog in commit message of promotion/propagation commits

[jlebon]

Idea from Dusty in #42 (comment):

Maybe in the future we can include the commit ids of all the commits the last merge. git shortlog --no-merges --pretty=format:"%h %s" -e LASTMERGE..HEAD would show a nice summary.

[jlebon]

Should probably also shorten the commit hash in the commit title and have the full length in the message: coreos/fedora-coreos-config#188