am, rebase: teach quiet option

am, rebase: teach quiet option

git-am and git-rebase are talkative scripts. Teach them to be quiet when
told, allowing them to speak only when they fail or experience errors.

The quiet option is maintained when git-am or git-rebase fails to apply
a patch. This means subsequent --resolved, --continue, --skip, --abort
invocations will be quiet if the original invocation was quiet.

Drop a handful of >&2 redirection; the rest of the program sends all the
info messages to stdout, not to stderr.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>