rev-parse: rev-parse: add --is-shallow-repository

Running `git fetch --unshallow` on a repo that is not in fact shallow
produces a fatal error message. Add a helper to rev-parse that scripters
can use to determine whether a repo is shallow or not.

Signed-off-by: Øystein Walle <oystwa@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>