replace: peel tag when passing a tag as parent to --graft

replace: peel tag when passing a tag as parent to --graft

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

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 as a parent in the
replacement object we create.

This created invalid objects, but the replace succeeded even if
it showed an error like:

error: object A is a tag, not a commit

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

While at it, let's also update an error message to make it
clearer.

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