I tested as follows
1. Installed https://secure.phabricator.com/P1819
(dropped the file into phabricator/extensions/)
2. viewed a commit with a Change-Id: field in the commit message.
3. Saw the Change-Id: in the property view, complete with a link to
gerrit.
Note: P1819 is pretty messy but the code in this change should be
pretty solid. Some of it was lifted from the differential conduit api.