R1:9092994e45c2

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…