Update the documentation of 'git shell' to mention the interactive
mode and COMMAND_DIR. Also provide a hint when interactive mode is not
available in the shell.
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Reviewed-by: Greg Brockman <gdb@MIT.EDU>
Signed-off-by: Junio C Hamano <gitster@pobox.com>