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

diff --cc Documentation/git-show-ref.txt
Simple merge

diff --cc builtin/show-ref.c
Simple merge