"git show-ref HEAD" used with "--verify" because the user is not
interested in seeing refs/remotes/origin/HEAD, and used with
"--head" because the user does not want HEAD to be filtered out,
i.e. "git show-ref --head --verify HEAD", did not work as expected.
* vp/show-ref-verify-head:
show-ref: remove a stale comment
show-ref: remove dead `if (verify)' check
show-ref: detect dangling refs under --verify as well
show-ref: move --quiet handling into show_one()
show-ref: allow -d to work with --verify
show-ref: accept HEAD with --verify