Fix bug in filename template parser

Summary: Quick fix for a bug in the parser. Before if the template included multiple `%Nd` expressions only the first would be parsed and all occurences of it replaced.

Test Plan:
Before:
{F6697593}
After:
{F6697592}

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D19814