object-file-convert: always make sure object ID algo is valid

In some cases, we zero-initialize our object IDs, which sets the algo
member to zero as well, which is not a valid algorithm number.  This is
a bad practice, but we typically paper over it in many cases by simply
substituting the repository's hash algorithm.

However, our new Rust loose object map code doesn't handle this
gracefully and can't find object IDs when the algorithm is zero because
they don't compare equal to those with the correct algo field.  In
addition, the comparison code doesn't have any knowledge of what the
main algorithm is because that's global state, so we can't adjust the
comparison.

To make our code function properly and to avoid propagating these bad
entries, if we get a source object ID with a zero algo, just make a copy
of it with the fixed algorithm.  This has the benefit of also fixing the
object IDs if we're in a single algorithm mode as well.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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