replace: peel tag when passing a tag first to --graft

replace: peel tag when passing a tag first to --graft

When passing a tag as the first argument to `git replace --graft`,
it can be useful to accept it and use the underlying commit as a
the commit that will be replaced.

This already works for lightweight tags, but unfortunately
for annotated tags we have been using the hash of the tag object
instead of the hash of the underlying commit.

Especially we would pass the hash of the tag object to
replace_object_oid() where we would likely fail with an error
like:

"error: Objects must be of the same type.
'annotated_replaced_object' points to a replaced object of type 'tag'
while 'replacement' points to a replacement object of type 'commit'."

This patch fixes that by using the hash of the underlying commit
when an annotated tag is passed.

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