git-submodule: add support for --rebase.

git-submodule: add support for --rebase.

'git submodule update --rebase' rebases your local branch on top of what
would have been checked out to a detached HEAD otherwise.

In some cases, detaching the HEAD when updating a submodule complicates
the workflow to commit to this submodule (checkout master, rebase, then
commit).  For submodules that require frequent updates but infrequent
(if any) commits, a rebase can be executed directly by the git-submodule
command, ensuring that the submodules stay on their respective branches.

git-config key: submodule.$name.rebase (bool)

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>