Some tweaks/fixes to roundtrip-js script

* Fix some lt/gt comparisons to account for the fact that
  dsr-end is exclusive and we should subtract -1 when comparing
  with a start value.
* Some other fixes.
* Fixes some incorrect semantic diff characterizations as
  syntactic diffs.

  Ex: en:List of Disney Films went from over 400 semantic diffs
  to about 72 semantic diffs (most of which are also actually
  syntactic diffs as well -- to be investigated).

Change-Id: Iaf327abaed953d8bea838103603b1090d450db44