Make old and new files when checking for changes by commit

Summary:
We have some false positives on commit changes checker.
I'm not sure if the reason is a difference between git diff and svn diff or something else but making this more robust doesn't harm anything.

We couldn't make the files from the whole changeset because I want to ignore context bigger than $num_lines to reduce rebase noise.

Test Plan:
Ran the method on diff which had false positive previously.
Ran the method on a diff with real change.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3107