Script for allowing web-platform-tests changes to be upstreamed.

This extends the upstream update script with steps for pushing
local changes to upstream. The general approach is to look for all
commits to the tests directory since the last sync, rewrite those
so they apply to upstream at the last sync point, then rebase onto
the sync commit, before creating and merging a PR for each in turn.
Corresponding upstream changes
Change that deletes a file
Update manifest script to allow handling of local changes in a non-checkout
Add updated manifest files