[libvirt] merge-commit push prohibition now in place, for master

[Jim Meyering]

Just today we noticed that it was possible to
push accidental merge commits to "master".
While the commit hook to prevent that was in place,
the config setting to enable the hook for "master"
was missing.  I've just done this on the server:

  git --git-dir /git/libvirt.git config hooks.denymerge.master true

so *now*, we should be protected from ourselves.