Fix query for changes using a label with a group operator.

The group operator is being ignored when searching for changes
with labels because the search index does not contain group
information.  This change will convert a group query into
mutiple user queries.

For example the query:
 'label:Code-Review=+1,group=heros'

Gets expanded to:
 'label:Code-Review=+1,user=superman OR label:Code-Review=+1,user=batman'

The latter is the actual query.

Bug: issue 3018
Change-Id: Ief5408ee4774f9491fe713b0089e76aa4cae4403
4 files changed