rebase/sync: use exit(1) for errors instead of exit(-1)

Callers don't actually see -1 (they'll usually see 255, but the exact
answer here is complicated).  Just switch to 1 as that's the standard
value tools use to indicate an error.

Change-Id: Ib712db1924bc3e5f7920bafd7bb5fb61f3bda44f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/233553
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>