do expensive diff after cheap parsing comments
do expensive diff after cheap parsing comments

In situations where the commit comments fail to parse, this will
make retrying cheaper as we won't be doing the expensive operation
of creating and storing a diff every time we unsuccessfully retry.

Test Plan: $ ./runtests

cs-shadow/phabricator-tools,aevri/phabricator-tools,aevri/phabricator-tools,kjedruczyk/phabricator-tools,cs-shadow/phabricator-tools,valhallasw/phabricator-tools,cs-shadow/phabricator-tools,bloomberg/phabricator-tools,kjedruczyk/phabricator-tools,kjedruczyk/phabricator-tools,bloomberg/phabricator-tools,bloomberg/phabricator-tools,bloomberg/phabricator-tools,aevri/phabricator-tools,cs-shadow/phabricator-tools,valhallasw/phabricator-tools,cs-shadow/phabricator-tools,kjedruczyk/phabricator-tools,aevri/phabricator-tools,aevri/phabricator-tools,valhallasw/phabricator-tools,bloomberg/phabricator-tools,kjedruczyk/phabricator-tools,valhallasw/phabricator-tools