[tracker] (7 commits) ...Merge branch 'wip/carlosg/sparql-fixes' into 'master'

[Sam Thursfield <sthursfield src gnome org>]

Summary of changes:

  ed9a786... libtracker-data: Check for variables in the most direct con (*)
  6b2a57d... tests: Add test for service in subselect (*)
  51cd345... libtracker-data: Handle empty GroupGraphPattern (*)
  d881546... tests: Add test for empty group patterns (*)
  e8309cb... libtracker-sparql: Differentiate property path elements by  (*)
  dce9bc2... tests: Add tests for property paths used from several graph (*)
  351b108... Merge branch 'wip/carlosg/sparql-fixes' into 'master'

(*) This commit already existed in another branch; no separate mail sent