Retire DownloadBuilder-hijack-way of building-demos

[rxaviers]

To save bytes and time, currently we're hijacking the jquery-ui checkout from download.jqueryui.com to copy demo files when building the demo.

Soon, Download Builder will handle multiple versions and the chance the get garbage (different checkout than expected) from its tmp dir is higher.

We may need to handle it by our own or find a consistent alternative.

[scottgonzalez]

This just bit us again. See #32.

[jzaefferer]

For the build-demos task we should just clone jQuery UI again instead of hijacking DB.

Since DB also has a bunch of git-related tasks, maybe we can build a little helper with we can include as a module in both DB and this repo's grunt file.

[jzaefferer]

Scott actually built a tool around git, though focusing on "a bunch of functions that make writing custom git commands or other git-based scripts much easier": https://github.com/scottgonzalez/node-git-tools

Could have something like Repo.fetch() which would handle cloning or fetching based on whether the repo exists.

[jzaefferer]

We can do a flat clone, since we don't need the full history to generate the demos. Just need the revision we're putting on the site. Use the --depth <depth> option to load only the specified number of revisions.

[jzaefferer]

This still sucks.