builtin-am: implement --skip
builtin-am: implement --skip

Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07), git-am
supported resuming from a failed patch application by skipping the
current patch. Re-implement this feature by introducing am_skip().

Helped-by: Stefan Beller <a7f3b1612c245dae606b3b9617b4f52187fddb65@google.com>
Signed-off-by: Paul Tan <ca5433e1cdd25b4a4001c54525dea593b33edc0c@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git