git-rebase.sh: Add check if rebase is in progress
git-rebase.sh: Add check if rebase is in progress

"git rebase --continue" and friends gave nonsense errors when there is no
rebase in progress.

Signed-off-by: Stephan Beyer <91ae524ae0f8ec8b44ccc7474652ac48e9556db7@gmx.net>
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