Provide api to get committed data in a hook

Summary:
We have a linter that needs to read data from the repo that isn't in the commit,
so that data isn't part of the svn transaction, thus not available in svnlook
cat --transaction. This change provides a method to get data from a committed
file.

Test Plan:
Successfully ran arc svn-hook-pre-commit with a linter that needs something
not in the transaction.

Reviewers: epriestley, jungejason, vrana

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2582