Merge branch 'en/rebase-no-keep-empty'

"git rebase" (again) learns to honor "--no-keep-empty", which lets the user to discard commits that are empty from the beginning (as opposed to the ones that become empty because of rebasing). The interactive rebase also marks commits that are empty in the todo. * en/rebase-no-keep-empty: rebase: fix an incompatible-options error message rebase: reinstate --no-keep-empty rebase -i: mark commits that begin empty in todo editor