describe: confirm that blobs actually exist

Prior to 644eb60bd0 (builtin/describe.c: describe a blob,
2017-11-15), we noticed and complained about missing
objects, since they were not valid commits:

  $ git describe 0000000000000000000000000000000000000000
  fatal: 0000000000000000000000000000000000000000 is not a valid 'commit' object

After that commit, we feed any non-commit to lookup_blob(),
and complain only if it returns NULL. But the lookup_*
functions do not actually look at the on-disk object
database at all. They return an entry from the in-memory
object hash if present (and if it matches the requested
type), and otherwise auto-create a "struct object" of the
requested type.

A missing object would hit that latter case: we create a
bogus blob struct, walk all of history looking for it, and
then exit successfully having produced no output.

One reason nobody may have noticed this is that some related
cases do still work OK:

  1. If we ask for a tree by sha1, then the call to
     lookup_commit_referecne_gently() would have parsed it,
     and we would have its true type in the in-memory object
     hash.

  2. If we ask for a name that doesn't exist but isn't a
     40-hex sha1, then get_oid() would complain before we
     even look at the objects at all.

We can fix this by replacing the lookup_blob() call with a
check of the true type via sha1_object_info(). This is not
quite as efficient as we could possibly make this check. We
know in most cases that the object was already parsed in the
earlier commit lookup, so we could call lookup_object(),
which does auto-create, and check the resulting struct's
type (or NULL).  However it's not worth the fragility nor
code complexity to save a single object lookup.

The new tests cover this case, as well as that of a
tree-by-sha1 (which does work as described above, but was
not explicitly tested).

Noticed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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