fast-import: add tests for tagging blobs

fast-import: add tests for tagging blobs

fast-import allows to create an annotated tag that annotates a blob,
via mark or direct sha1 specification.

For mark it works, for sha1 it tries to read the object. It tries to
do so via read_sha1_file, and then checks the size to be at least 46.

That's weird, let's just allow to (annotated) tag any object referenced
by sha1. If the object originates from our packfile, we still fail though.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>