Specifically, require that the left branch (first restult of git
show -s --format=format:%P) is a signed merge commit, instead of
allowing either. This is fine for now, but might need to be relaxed
in the future.
Also fixes an out-of-file-descriptors issue by holding too many
open FDs writing to /dev/null
Any attacker who managed to make an evil commit that changed something in the
contrib/verify-commits/ directory could just as easily remove the warning
and/or modify it to not display the evil commits; telling the user to check
those commits specifically misleads them into checking just those commits
rather than the script itself.
