rebase: support --no-autostash

This is documented as an option but we don't actually accept it.
Support it so that it is possible to override the "rebase.autostash"
config variable.

Reported-by: Daniel Hahler <genml+git-2014@thequod.de>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-rebase.sh b/git-rebase.sh
index 55da9db..95a4593 100755
--- a/git-rebase.sh
+++ b/git-rebase.sh
@@ -14,7 +14,7 @@
  Available options are
 v,verbose!         display a diffstat of what changed upstream
 q,quiet!           be quiet. implies --no-stat
-autostash!         automatically stash/stash pop before and after
+autostash          automatically stash/stash pop before and after
 fork-point         use 'merge-base --fork-point' to refine upstream
 onto=!             rebase onto given branch instead of upstream
 p,preserve-merges! try to recreate merges instead of ignoring them
@@ -292,6 +292,9 @@
 	--autostash)
 		autostash=true
 		;;
+	--no-autostash)
+		autostash=false
+		;;
 	--verbose)
 		verbose=t
 		diffstat=t