Fix db:check-schema in forks and shallow clones

The `regenerate-schema` script attempts to find the merge base between
the source and branch SHA to determine the right `db/structure.sql` to
apply the migrations in the merge request. However, to get the merge
base, we need the target SHA and the history between the source and
target SHAs. Instead of downloading it via a curl request, we fetch the
target branch and check out the `db/structure.sql` from there.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/216214