Set gerrit change.conflictsPredicateEnabled to true

This is a new config option for Gerrit 3.5. While it defaults to true we
set it explicitly to true to avoid any changes in behavior should that
default change eventually with newer Gerrit. They note this is expensive
to calculate, but our users rely on it and it hasn't caused us problems
yet. We can always explicitly disable it in the future if that becomes
necessary.

Change-Id: Idc002810de2d848af043978894ef9dc194ac5b6a

playbooks/roles/gerrit/templates/gerrit.config.j2

@@ -87,6 +87,8 @@
 	move = false
 # Keep pre-3.4 behavior
 	mergeabilityComputationBehavior = API_REF_UPDATED_AND_CHANGE_REINDEX
+# Explicitly enable this to avoid problems if the default changes
+	conflictsPredicateEnabled = true
 [receive]
 	maxObjectSizeLimit = 100 m
 	enableSignedPush = false