ls-files usage doesn't work on massive repositories

When reformatting an entire repo, if there are "too many" files, the git ls-files commands fail with argument-list-too-long problems when slinging them around the various parts.

• git-workarea should expose a way to checkout files that came from ls-files elsewhere to avoid the internal problem it has in GitWorkArea::checkout()
• git ls-files needs a --stdin (and -z) so that we can pass paths in rather than through the command line.

Cc: @brad.king