dashboard/app: show Mercurial page at /hg/

Change-Id: If5e77adc3766609b8f37423d0f132bcceff704f8
Reviewed-on: https://go-review.googlesource.com/1359
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>