something like git-for-each-ref's --format for commits?

[Brian Swetland <swetland@google.com>]

I'm trying to obtain various information about commits from a script.
The --format option to git-for-each-ref would be perfect, except that
it only operates on refs, not arbitrary commit objects.

The --pretty=format:... option for git-log, etc has the annoying
property of giving you <unknown> instead of an empty string if an
item isn't example (say if you use %b on a commit with no body).

Am I missing some obvious solution here?  

Thanks,

Brian