ci-db-backcomp.sh is not branch-aware

Created by: uwedeportivo

see for example https://buildkite.com/sourcegraph/sourcegraph/builds/68763#f4388355-891f-4ace-be26-4b4401a103b3

if a DB schema change lands in master after a release branch was cut, then this check fails in the branch, because it compares HEAD of branch with latest in master.

make this check branch-aware