This adds a "Git" dashboard at "/git/", which has its own namespace for
datastore and memcache. Once we have pushed out the new git repositories
we will spin up a commit watcher and builders that point to the Git
dashboard.
Once we are ready to switch the dashboard over to the new Git builders,
the (*Dashboard).Context method will be changed to return "Git"
as the default namespace. The old builders will be retired, and the
new builders will be configured to report to "/" instead of "/git/".
At that point all our old data will still be available in the default
namespace, but hidden from view.
LGTM=dsymonds, cmang
R=bradfitz, cmang, dsymonds, adg
CC=golang-codereviews
https://golang.org/cl/183050043
This CL moves code from code.google.com/p/dvyukov-go-perf-dashboard,
which was previously reviewed.
LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/96180043
