For the 'conflicts' operator return only changes that actually conflict

So far the 'conflicts:<change>' query operator returns all changes that
touch at least one file that was also modified by the specified change.
This means all changes that potentially conflict with the specifed
change are returned.

Now we check the mergeability for the changes and only return those
changes that actually conflict. This means now the 'Automatically
resolve conflicts' project setting is taken into account and if the
content merge succeeds a change is not returned.

A new persistent cache is added to cache for two commits and a given
submit type whether the commits are conflicting. This ensures that the
expensive mergeability check for two changes is only done once.

Change-Id: I1d6a3c431ed679f786e589bbed3aed52fe439895
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-server/src/test/java/com/google/gerrit/server/index/FakeQueryBuilder.java

@@ -26,7 +26,7 @@ public class FakeQueryBuilder extends ChangeQueryBuilder {
         new FakeQueryBuilder.Definition<ChangeData, FakeQueryBuilder>(
           FakeQueryBuilder.class),
         new ChangeQueryBuilder.Arguments(null, null, null, null, null, null,
-          null, null, null, null, null, null, null, indexes),
+          null, null, null, null, null, null, null, indexes, null, null),
         null);
   }