[UnindexedFileIndexer] Loop optimizations (avoid detach, invariants)

Summary:
Avoid detach of QStringList in for loop.
Move invariant IndexingLevel out of the loop.

Test Plan: make

Reviewers: Baloo, ngraham, astippich, poboiko, broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21572