Rework file numbering

Summary:
Previously we looped though every match and checked the filesystem every
iteration. Now we do it only once which should result in better performance. I
also added a test case that shows that the old method was broken, too.

Test Plan: run tests

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D28315