git-rebase-insert: new command

Because we don't want to depend on Magit this has to be uglified a bit.
Because we also don't want to depend on Dash yet this has to be uglified
even more.  Closes magit/magit#1523.
Merge branch 'master' into next

Conflicts:
	git-rebase-mode.el
git-rebase-insert: use --if-let