am: --rebasing

am: --rebasing

The new option --rebasing is used internally for rebase to tell am that
it is being used for its purpose.  This would leave .dotest/rebasing to
help "completion" scripts tell if the ongoing operation is am or rebase.

Also the option at the same time stands for --binary, -3 and -k which
are always given when rebase drives am as its backend.

Using the information "am" leaves, git-completion.bash tells ongoing
rebase and am apart.

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