apply: diagnose incomplete submodule object name better

"git am -3" uses this function to build a tree that records how the
preimage the patch was created from would have looked like.  An
abbreviated object name on the index line is ordinarily sufficient
for us to figure out the object name the preimage tree would have
contained, but a change to a submodule by definition shows an object
name of a submodule commit which our repository should not have, and
get_sha1_blob() is not an appropriate way to read it (or get_sha1()
for that matter).

Use get_sha1_hex() and complain if we do not find a full object name
there.

We could read from the payload part of the patch to learn the full
object name of the commit, but the primary user "git rebase" has
been fixed to give us a full object name, so this should suffice
for now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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