After refactoring ReadTreeTest the tests failed for filesystems
with coarse modification time granularity. This is fixed by
explicitly telling the repo to reread the index after we build
a new index.
Additionally the test testDirectoryFileSimple was simplified
by using buildTree() instead of misusing GitIndex to construct
trees.
Change-Id: I20d2f097491e4cc8c657a696beabc7026b485017 Signed-off-by: Christian Halstrick <christian.halstrick@sap.com> Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>