Merge branch 'jn/shell-disable-interactive'

When the interactive access to git-shell is not enabled, we issue a
message meant to help the system admininstrator to enable it. Add
an explicit way to help the end users who connect to the service by
issuing custom messages to refuse such an access.

* jn/shell-disable-interactive:
  shell: new no-interactive-login command to print a custom message
  shell doc: emphasize purpose and security model