builtin/cat-file: deduplicate logic to iterate over all objects

builtin/cat-file: deduplicate logic to iterate over all objects

Pull out a common function that allows us to iterate over all objects in
a repository. Right now the logic is trivial and would only require two
function calls, making this refactoring a bit pointless. But in the next
commit we will iterate on this logic to make use of bitmaps, so this is
about to become a bit more complex.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>