builtin/replace: teach listing using short, medium or full formats

By default when listing replace refs, only the sha1 of the
replaced objects are shown.

In many cases, it is much nicer to be able to list all the
sha1 of the replaced objects along with the sha1 of the
replacment objects.

And in other cases it might be interesting to also show the
types of the replaced and replacement objects.

This patch introduce a new --format=<fmt> option where
<fmt> can be any of the following:

	'short': this is the same as when no --format
		option is used, that is only the sha1 of
		the replaced objects are shown
	'medium': this also lists the sha1 of the
		replacement objects
	'full': this shows the sha1 and the type of both
		the replaced and the replacement objects

Some documentation and some tests will follow.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>