replace: make sure --edit results in a different object

It's a bad idea to create a replace ref for an object
that points to the original object itself.

That's why we have to check if the result from editing
the original object is a different object and error out
if it isn't.

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

This will be useful in a following commit when we will
want to check if the ref already exists before we let the
user edit an object.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
replace: die early if replace ref already exists

If a replace ref already exists for an object, it is
much better for the user if we error out before we
let the user edit the object, rather than after.

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

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
replace: add --edit to usage string

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Documentation: replace: describe new --edit option

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