[Arcanist] collect svn and bookmark info for Mercurial

Summary:
This increases the amount of information arc diff collects in
Mercurial repositories. IN particular, it collects the active
bookmark, if there is one, and svn information in hgsubversion
repositories.

The Phabricator half of this is https://secure.phabricator.com/D2897

Test Plan:
[14:06:59 Sat Jun 30 2012] dschleimer@dev4022.snc6 ~/www-hg
www-hg 2941 $ ~/devtools/arcanist/bin/arc diff --only --conduit-uri http://phabricator.dschleimer.dev4022.facebook.com
HipHop Notice: Undefined index: 1 in /data/users/dschleimer/devtools/arcanist/src/repository/api/ArcanistMercurialAPI.php on line 708
Created a new Differential diff:

Diff URI: http://phabricator.dschleimer.dev4022.facebook.com/differential/diff/126/

Included changes:

A       foo

HipHop Notice: Undefined index: 1 in /data/users/dschleimer/www-hg/lib/arcanist/arcanist/FacebookArcanistConfiguration.php on line 81
mcproxy on this server is out of date
version: , expected version:
please restart (http://fburl.com/1787362)

[14:07:46 Sat Jun 30 2012] dschleimer@dev4022.snc6 ~/www-hg
www-hg 2941 $ echo '{"diff_id": 126}' | ~/devtools/arcanist/bin/arc call-conduit differential.getdiff --conduit-uri http://phabricator.dschleimer.dev4022.facebook.com | json_pretty | egrep -i 'bookmark|sourcecontrol'

"bookmark": "bar",
"sourceControlBaseRevision": "svn+ssh://tubbs/svnroot/tfb/trunk/www@583442",
"sourceControlPath": "svn+ssh://tubbs/svnroot/tfb/trunk/www",
"sourceControlSystem": "hg",

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1331

Differential Revision: https://secure.phabricator.com/D2896