Add JSON output and support for all refs/ to +refs about gitiles HOT 6 CLOSED

The current refs listing provided by +refs appears to be limited to refs/heads/ and
refs/tags/, and doesn't support JSON output. It would be useful to also be able to
list other refs/. Maybe +refs could default to the current listing, but could be extended
with an 'ls-remote'-style filter pattern as the path. If the JSON output could also
include the commit hash of each ref, like ls-remote, that would be perfect.

For instance, https://chromium.googlesource.com/external/msysgit/git/+refs only shows
the branches that we copy into refs/heads/ but there are actually a lot more branches
that are mirrored from upstream into refs/remotes/upstream/, so to get all upstream
branches:
https://chromium.googlesource.com/external/msysgit/git/+refs/remotes/upstream/*

or all 'master' branches (both refs/heads/ and refs/remotes/upstream/):
https://chromium.googlesource.com/external/msysgit/git/+refs/*/master

(FWIW, this is needed by some Chrome scripts which currently shell out to a local git
to run ls-remote, which prevents us from running those scripts on appengine.)

Already available with format=TEXT:
$ curl -s 'https://chromium.googlesource.com/external/msysgit/git/+refs?format=TEXT'
| grep -c refs/remotes
36

I guess we could do JSON if you really want.

Ah, thanks, I didn't try that format. That's sufficient for my purposes.

Works great. Feel free to close.

I'll leave this bug open as the description is still accurate, we have neither JSON
output nor a way to list all refs from the HTML view :)