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

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

Since e481af06 (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 <redacted>
Signed-off-by: Clemens Buchacher <redacted>
Helped-by: Jorge Nunes <redacted>
Signed-off-by: Junio C Hamano <redacted>