Is there a fast and easy way to find out which of a set of SHA1 ids
refer to commits that (still) exist in a repository?

This is for use in gitk and there could be several ids, so I'd prefer
to avoid a fork/exec per id.  I could do a git cat-file -t $id for
each id, but that's a fork/exec per id.  git rev-parse doesn't check
whether an id actually refers to an existing commit, so it isn't the
answer.

What I want to be able to do is to cache the condensed topology
information that gitk uses for working out next/previous tags.  But
when I read in the cache I need to be able to know if the topology
includes commits that used to exist but have now been removed.  Hence
my question.

Thanks,
Paul.
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