We read from the marks file and keep only marked commits, but in
order to find the type of object, we are parsing the whole thing,
which is slow, specially in big repositories with lots of big files.

There's no need for that, we can query the object information with
sha1_object_info().

Before this, loading the objects of a fresh emacs import, with 260598
blobs took 14 minutes, after this patch, it takes 3 seconds.

This is the way fast-import does it. Also die if the object is not
found (like fast-import).
Signed-off-by: NFelipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>