Refactor repository reparse scripts to be more useful

Summary:
Splitting up D960 a bit, see that for context.

We currently have two scripts, "parse_one_commit.php" and
"reparse_all_commit_messages.php", but they're sort of silly and you can't do
certain things with them. Replace them with one script which is more flexible
and can do specific reparse steps on individual commits or entire repos.

I left the old scripts as stubs since I think there are some FB wiki docs and
stuff that mention them. I'll delete them in a month or whenever I remember or
something.

Test Plan: Ran "reparse.php" with various arguments, including vs-one-commit,
vs-repository, with --trace, and against different types of repos.

Reviewers: Makinde, jungejason, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 964