This is a little tool to mount a Github repository locally, so that you can
browse it from the command line without cloning it.

To mount a repo REPO by github user USERNAME in the directory DIR:
$ cd src
$ ./githubfs.rb USERNAME REPO DIR &

You will be asked for your github username and password. (githubfs will
only be able to access public repositories; it only needs to authenticate
to get more generous rate limits.)
Then you can cd into DIR and poke around. To unmount, kill the githubfs process
and run "unmount DIR".

TO DO:
- speed it up by memoizing all the file system operations, and maybe
  speculatively listing subdirectories, etc.
- write a version of git-checkout that takes a ref, and writes it to a special
  file, and GithubFS checks that file for which ref to use before each operation
- write a verison of git-log that reads history from the github API