mkosi: Make sure we don't hide errors from git merge-base

mkosi: Make sure we don't hide errors from git merge-base

Currently if git merge-base fails we'll hide the error and exit with
exit status 0. Let's make we only exit early if git merge-base exits
with 1 which indicates the current commit is not on the target branch.
Any other error is considered fatal.

(cherry picked from commit 2fe6ad5a64bca187712708d41269091c91500a35)