gitpipe: Implement iterator for catfile object step

The pipeline steps are currently easy to misuse: the structures returned
on the channel all have an `Err` field, which MUST be checked by the
caller. It's easy enough to forget if one isn't prepared to do so.

Refactor the catfile object step to instead return a
CatfileObjectIterator. This iterator follows the typical Go idiom of
iterators with `Next()`, `Err()` and `Result()` functions. Given that
the iterator is an interface, this also makes it easily possible for
callers to replace it with their own adapter.

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