Add new method IndexDiff#getPathsWithIndexMode

Get the list of paths that have the given file mode.

This helps EGit to efficiently determine which modified files are
symlinks and should be shown with a symlink icon in the staging view.

Bug: 429302
Change-Id: Id15f0c6f265667f5b8b57cc2d9f97de568371919
Signed-off-by: Axel Richard <axel.richard@obeo.fr>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>