Fix: wrongly throw 'commit already exists' exception

The reason is the ReplaceRequest.validate() should compare
to 'newCommit' of ReplaceRequest instance, else when
ReceiveCommits.autoCloseChanges() working on the second one
of a series of commits, the 'commit already exists(in the project)'
will be thrown wrongly.

Fixed.

Bug: issue 2607
Change-Id: Ib4db351d39f85d57ccf532da4d6422bdcbb88105