am --abort: keep unrelated commits since the last failure and warn

am --abort: keep unrelated commits since the last failure and warn

After making commits (either by pulling or doing their own work) after a
failed "am", the user will be reminded by next "am" invocation that there
was a failed "am" that the user needs to decide to resolve or to get rid
of the old "am" attempt.  The "am --abort" option was meant to help the
latter.  However, it rewinded the HEAD back to the beginning of the failed
"am" attempt, discarding commits made (perhaps by mistake) since.

Signed-off-by: Junio C Hamano <gitster@pobox.com>