Add repositories to fulltext search index.

Summary:
This implements a simplistic PhabricatorRepositoryFulltextEngine
Currently only the repository name, description, timestamps and
status are indexed.

Note: I had to change the search index workflow to disambiguate
PhabricatorRepository from PhabricatorRepositoryCommit

Test Plan:

• ran ./bin/search index --type PhabricatorRepository --force
  • searched for some repositories. Saw reasonable results matching on either title or description.
• Edited a repository in the web ui
  • Added unique key words to the repo description.
  • I was then able to find that repo by searching for the new keywords.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Tags: #search, #diffusion

Differential Revision: https://secure.phabricator.com/D17300