Allow creation of arbitrary git-shell commands

Allow creation of arbitrary git-shell commands

This provides a mechanism for the server to expose custom
functionality to clients.  My particular use case is that I would like
a way of discovering all repositories available for cloning.  A
client that clones via

  git clone user@example.com

can invoke a command by

  ssh user@example.com $command

Signed-off-by: Greg Brockman <gdb@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>