Previously each group was rendered asynchronously, i.e. deferring
control between each group render. This caused the total render time to
be ~10x slower than totally synchronous. But the advantage of this
approach was that the content at the start of the diff would be visible
as soon as possible.
This change finds a compromise by combining processing/rendering the
diff groups in larger synchronous batches which are based on the number
of lines in the groups.
Change-Id: I5d42700962f51bea0dedb05763a1d8eb1c32bfd1
0f0c94189f