This method tried to iterate spurious files which may exist in the
.git/refs folder, e.g. on Mac a .DS_Store may have been created there by
inspecting the folder using the finder application. This led to a
NotDirectoryException when deleteEmptyRefsFolders tried to create an
iterator for such a file entry. Skip files contained in the refs folder
to ensure the method only tries to iterate contained folders but not
files.
Change-Id: I5f31e733072a35db1e93908a9c69a8891ae5c206
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>