submodule: add --no-fetch parameter to update command

submodule: add --no-fetch parameter to update command

git submodule update --no-fetch makes it possible to use git submodule
update in complete offline mode by not fetching new revisions.

This does make sense in the following setup:

* There is an unstable and a stable branch in the super/master repository.
* The submodules might be at different revisions in the branches.
* You are at some place without internet connection ;)

With this patch it is now possible to change branches and update
the submodules to be at the recorded revision without online access.

Another advantage is that with -N the update operation is faster, because fetch is checking for new updates even if there was no fetch/pull on the super/master repository since the last update.

Signed-off-by: Fabian Franz <git@fabian-franz.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>