Merge branch 'jk/cherry-pick-0-mainline'

"git revert -m 0 $merge_commit" complained that reverting a merge
needs to say relative to which parent the reversion needs to
happen, as if "-m 0" weren't given.  The correct diagnosis is that
"-m 0" does not refer to the first parent ("-m 1" does).  This has
been fixed.

* jk/cherry-pick-0-mainline:
  cherry-pick: detect bogus arguments to --mainline

diff --cc builtin/revert.c
Simple merge