rebase: return non-zero error code if format-patch fails

Since e481af0 (rebase: Handle cases where format-patch fails) we
notice if format-patch fails and return immediately from
git-rebase--am. We save the return value with ret=$?, but then we
return $?, which is usually zero in this case.

Fix this by returning $ret instead.

Cc: Andrew Wong <andrew.kw.w@gmail.com>
Signed-off-by: Clemens Buchacher <clemens.buchacher@intel.com>
Helped-by: Jorge Nunes <jorge.nunes@intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Clemens Buchacher

git-rebase--am.sh

@@ -78,7 +78,7 @@ else
 
 		As a result, git cannot rebase them.
 		EOF
-		return $?
+		return $ret
 	fi
 
 	git am $git_am_opt --rebasing --resolvemsg="$resolvemsg" \