Improve 'git-crypt init' usability

  * Correctly check for existence of HEAD (use 'git rev-parse' instead
    of 'git show-ref').  Fixes bug where hard reset might be skipped
    after running 'git init'.
  * Don't require working directory to be clean if HEAD doesn't exist.
    (If HEAD doesn't exist, we won't be hard resetting so the working
    directory doesn't need to be clean.)
  * Overwrite existing git config values (instead of --add'ing them) so
    'git-crypt init' can be idempotent.
  * In the error message for a disrty working directory, advise user to
    commit changes or 'git stash' them.