Add commits to the Phabricator search index

Summary:
create the indexer for commit

Test Plan:
run the reindex_one_commit.php against one existing commit
and it is returned in search result on the webpage; run parse_one_commit
against another commit; modified reindex_everything.php to let it only
parse one commit after loading all commits.

Reviewed By: epriestley
Reviewers: epriestley, aran
CC: aran, jungejason, epriestley, debow
Differential Revision: 490