Building on existing support for asynchronous diff processing, the
rendering stage is now also asynchronous. The `emitGroup` method of
gr-diff-builder (which constructs a DOM fragment and attaches it to the
document) is now called whenever the processor emits a new group, rather
than waiting for all groups to be available and looping over them.
Adds support for canceling the processor, and removes a behavior where
the diff was being re-rendered needlessly, causing visible flicker.
Updates a test that broke when rendering became asynchronous.
Change-Id: I37fcd65efc8c901b85fc93e91611c022edc10dc4
db881fa0ad