[arc svn-hook-pre-commit] Access working copy

Summary:
Creates a new hook API that can be used to interface with
SVN/Git/Mercurial in the context of a commit hook. Currently only adds a
function to read the modified file data in a Subversion commit hook.

An object of this API is created in the SvnHookPreCommitWorkflow and
passed on the Lint Engine which then uses it to access current file
data, of the way the APIs seem to be structured); linters use the
getData function which is essentially a wrapper around the engine's
call, with another layer of caching.

Task ID: #770556

Blame Rev:

Test Plan:

• Create a local svn repository and add a minimal hook to run the local version of arc to test commits

(http://phabricator.com/docs/arcanist/article/Installing_Arcanist_SVN_Hooks.html)

• Create a temporary repository that can trigger any of the linters available, and test against a temporary linter by committing against the test repository: the linter should be able to access all required files by using loadData/getData in the LintEngine and Linter.

Revert Plan:

Tags: lint, svn-hook-pre-commit

Reviewers: jungejason, asukhachev, epriestley, aran

Reviewed By: epriestley

CC: aran, jungejason, epriestley, kunalb, asukhachev

Differential Revision: https://secure.phabricator.com/D1256