R1:9e2f263bb49c

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…