More bug fixes and cleanup in roundtrip-test.js diffing code

* Removed unnecessary DOM walking when not required
* Ensured that matching nodes in a desired DSR range are not lost.
* More cleanup possible: global 'waitingForEndMatch' flag could
  possibly be eliminated.

* Test case: node roundtrip-test.js Jehan_de_Bethancourt
  This used to report a semantic diff in place of a syntactic diff

Change-Id: I0267c7583e4c960a3eb10e9adb7cd04021da4621
Merge "More bug fixes and cleanup in roundtrip-test.js diffing code"