The build for Sapling's isl package has changed:
* Instead of being a bunch of .js files installed along-side the main
executable, isl is now one isl-dist.tar.xz file extracted at
run-time by the 'sl web' command.
* To configure the 'node' executable used by 'sl web', set the
web.node-path Sapling setting by changing the default configs
embedded in the sl executable.
