Merge attached revisions and commits in task detail

Summary:
With freetext fields, most tasks would have attached revisions when they have attached commits.
See T1048 for example.
Merge these two fields together.

Test Plan:
Displayed task with commit and unrelated revision.
Displayed task with commit and related revision:
{F39394,size=full}

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5601