fix handling of sub modules in git rely on ls-files --recurse-submodules, there since end of 2016
Summary: before this fix, submodules lead to duplicate files in the file tree for a project
Test Plan:
test e.g.
https://github.com/christoph-cullmann/cullmann.io
without patch: duplicated files, missing sub-modules
with patch: no duplicates + sub-module files are there
Subscribers: kwrite-devel
Tags: Kate
Differential Revision: https://phabricator.kde.org/D20343