Speed up the RepositoriesViewContentProvider

Speed up the RepositoriesViewContentProvider

Some enhancements for the working tree part:

* It's not necessary to sort in the content provider since we have a
  RepositoriesViewSorter. Besides, that sorting was very inefficient
  because it repeatedly queried File.isDirectory(), which is an
  expensive call on Windows.
* Short-cut the hasChildren() call for files and folders. This gives
  a significant speedup for large directories; the old implementation
  actually read the whole directory for a folder, but using a
  DirectoryStream we can stop reading after the first entry.
* Use Files.walkFileTree() with depth 1 to get the children and their
  attributes in one go on Windows. On Linux, this shouldn't make any
  difference; there'll be one call to get file attributes to determine
  isDirectory() anyway, but on Windows, this saves an extra call to
  getAttributes() per child.

Change-Id: Ie0bf1d89167f4399f72e4f9372aeea3dff24a320
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>