Merge branch 'rh/peeling-tag-to-tag'

Merge branch 'rh/peeling-tag-to-tag'

Make "foo^{tag}" to peel a tag to itself, i.e. no-op., and fail if
"foo" is not a tag.  "git rev-parse --verify v1.0^{tag}" would be a
more convenient way to say "test $(git cat-file -t v1.0) = tag".

* rh/peeling-tag-to-tag:
  peel_onion: do not assume length of x_type globals
  peel_onion(): add support for <rev>^{tag}