Merge branch 'jk/rev-list-empty-input' into maint

"git log --tag=no-such-tag" showed log starting from HEAD, which has been fixed---it now shows nothing. * jk/rev-list-empty-input: revision: do not fallback to default when rev_input_given is set rev-list: don't show usage when we see empty ref patterns revision: add rev_input_given flag t6018: flesh out empty input/output rev-list tests