Merge branch 'stable-4.11' into stable-5.0

Merge branch 'stable-4.11' into stable-5.0

* stable-4.11:
  Silence API warning for new ObjectIdSerializer introduced in 4.11.1
  Ignore API warnings
  Fix photon target platform to use photon version of org.eclipse.osgi
  Update Photon orbit repository to R20180606145124
  Suppress warning for trying to delete non-empty directory
  Fix fetching with duplicate ref updates
  Fetch(Process): should tolerate duplicate refspecs
  FetchCommandTest: test add/update/delete fetch

Change-Id: I2497f99b7d0686a0e0f773c39c8725e813ce9d4a
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>