t: add lib-loose.sh

t: add lib-loose.sh

This commit adds a shell library for writing raw loose objects into the
object database. Normally this is done with hash-object, but the
specific intent here is to allow broken objects that hash-object may not
support.

We'll convert several cases that use "hash-object --literally" to write
objects with invalid types. That works currently, but dropping this
dependency will allow us to remove that feature and simplify the
object-writing code.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>