vlt.git.git_install

  GIT_PULL - pull changes to a git repository

  B = vlt.git.git_pull(DIRNAME, REPOSITORY)

  'Install' is our term for forcing the local directory DIRNAME to match the
  remote REPOSITORY, either by cloning or pulling the latest changes. Any files
  in the local directory DIRNAME that don't match the remote REPOSITORY are deleted.

  If DIRNAME does not exist, then the repository is cloned.
  If DIRNAME exists and has local changes, the changes are stashed and the
     directory is updated by pulling.
  If the DIRNAME exists and has no local changes, the directory is updated by
     pulling.

  Note: if you have any local changes, vlt.git.git_install will stash them and warn the user.

  B is 1 if the operation is successful.