rebase: use our standard error return value

rebase: use our standard error return value

Git uses −1 to signal an error. The builtin rebase converts these to
+1 all over the place using !! (presumably because the in the scripted
version an error was signalled by +1). This is confusing and clutters
the code, we only need to convert the value when the function returns.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>