Merge branch 'db/show-ref-head'

The "--head" option to "git show-ref" was only to add "HEAD" to the list of candidate refs to be filtered by the usual rules (e.g. "--heads" that only show refs under refs/heads). Change the meaning of the option to always show "HEAD" regardless of what filtering will be applied to any other ref (this is a backward incompatible change, so I may need to add an entry to the Release Notes). * db/show-ref-head: show-ref: make --head always show the HEAD ref