Add a handler for /+describe similar to "git describe"

Options parallel options to "git describe", e.g.
$ curl http://gitiles/repo/+describe/deadbeef?contains&all&format=JSON
master~3

Only JSON and TEXT formats are supported, there is no HTML template.
For now, only --contains is supported, using JGit's NameRevCommand
(since "git describe --contains" calls "git name-rev" internally).

Change-Id: Ia71cb546645f93eb19eac4db69103835e46ae678