When examining merge commits in our Git hooks, only check the first parent

When examining merge commits in our Git hooks, only check the first parent

This does two things:

  1. We can properly validate log entries in merge commits.
  2. We don't check commits that were merged in from other branches.

* build-aux/git-hooks/commit-msg-files.awk (get_commit_changes): Get
the changes compared to the first parent.

* build-aux/git-hooks/pre-push: Only get the first parent of merge
commits when returning the rev-list, and only check "master" or
"emacs-NN" branches.