Change: correctly rectify no-change

Modify `Change.rectify()` to not make any changes to the Change instance
(most notably to not modify the `checkpoint` field) when the tracked
model instance was not changed.

This should improve the performance of change replication as it reduces
the number of unnecessary replications.

For example, before this commit, every run of `rectifyAll` would
trigger a full sync of all clients, because all change instances would
be moved to the current checkpoint.

common/models/change.js

@@ -192,7 +192,8 @@ module.exports = function(Change) {
       if (rev) {
         // avoid setting rev and prev to the same value
         if (currentRev === rev) {
-          change.debug('rev and prev are equal (not updating rev)');
+          change.debug('rev and prev are equal (not updating anything)');
+          return cb(null, change);
         } else {
           change.rev = rev;
           change.debug('updated revision (was ' + currentRev + ')');

test/change.test.js

@@ -200,6 +200,23 @@ describe('Change', function() {
         });
       }
     });
+
+    it('should not change checkpoint when rev is the same', function(done) {
+      var test = this;
+      var originalCheckpoint = change.checkpoint;
+      var originalRev = change.rev;
+
+      TestModel.checkpoint(function(err, inst) {
+        if (err) return done(err);
+
+        change.rectify(function(err, c) {
+          if (err) return done(err);
+          expect(c.rev, 'rev').to.equal(originalRev); // sanity check
+          expect(c.checkpoint, 'checkpoint').to.equal(originalCheckpoint);
+          done();
+        });
+      });
+    });
   });
 
   describe('change.currentRevision(callback)', function() {