Add missing group_get_object() function.

This fix sla #4036 (unable to use the global search with mediawiki)

Change-Id: I9104e08c091d0ebc1b0efe030a61efc5f1b92195
Merge commit 'refs/changes/60/560/1' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  Add missing group_get_object() function.

Change-Id: Ibc9acff2b477a71548fcc0c655a77750c1e85c2a
Tuleap 6.1.99.29

Change-Id: I5e701645b6d70b01bb2fbed2e302da1845dd5592