Unnecessary double-purge if modified file has the same URL

Problem/Motivation

File URL is purged twice on update.

Steps to reproduce

When updating a file, this module purges both the original and new URL of the file, which is great! However, if these are the same, we should only purge the URL once. This will be just a tiny micro-optimization but still seems worth it :)

Proposed resolution

Compare the old and new file URLs before issuing two purges.