GitLab's online editor doesn't seem to add tab characters

Trufflehog's --since_commit didn't account well for branches being
created, destroyed, or merged. Rather than continuing to fix
TruffleHog itself, we change strategies and use git to compute the new
commit hashes. Then, we feed those hashes one at a time to TruffleHog,
limiting it's history by using max_depth of 1.

This uses trufflehog to scan for secrets, and rejects them if present. It will
also check the HEAD reference for various configuration options, which will be
used during the scanning. If all else fails, a push option can force this hook
to be ignored.