Improve path handling in diff parsing

This MR fixes the following:

1. git diff can return file names escaped in octal bytes and our regexp couldn't match (being wrapped in a double quotes), so we fix the regexp and unescape the file name
2. We were storing /dev/null as a old/new path, but it's not being used in the application, so we change this behavior.
3. A bug that was raised by 1.