Teach git-describe how to run name-rev

Teach git-describe how to run name-rev

Often users want to know not which tagged version a commit came
after, but which tagged version a commit is contained within.
This latter task is the job of git-name-rev, but most users are
looking to git-describe to do the job.

Junio suggested we make `git describe --contains` run the correct
tool, `git name-rev`, and that's exactly what we do here.  The output
of name-rev was adjusted slightly through the new --name-only option,
allowing describe to execv into name-rev and maintain its current
output format.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>