Add merge feature into /changes REST endpoint

Allow user to create a merge in Gerrit via POST /changes REST API.
By adding a "merge" attribute which could be a SHA1, a branch name or a
tag name, etc. in the post JSON, gerrit will create a merge commit
instead of an empty commit. If there are conflicts, the response will
be rejected with a MergeConfictException which contains conflict
message.

Add dry run end point into GET
/projects/{project}/branches/{branch}/mergeable with query parameters
source (required) and strategy (optional) and return a
MergeableInfo entity.

Change-Id: I8f45f324704b3ff3eb20cb57c6e3bd75f2bf60ef

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/PushOneCommit.java

@@ -206,6 +206,11 @@ public class PushOneCommit {
     }
   }
 
+  public void setParent(RevCommit parent) throws Exception {
+    commitBuilder.noParents();
+    commitBuilder.parent(parent);
+  }
+
   public Result to(String ref) throws Exception {
     commitBuilder.add(fileName, content);
     return execute(ref);