oidtree: extend iteration to allow for arbitrary return codes

oidtree: extend iteration to allow for arbitrary return codes

The interface `cb_each()` iterates through a crit-bit tree and calls a
specific callback function for each of the contained items. The callback
function is expected to return either:

  - `CB_CONTINUE` in case iteration shall continue.

  - `CB_BREAK` to abort iteration.

This is needlessly restrictive though, as callers may want to return
arbitrary values and have them be bubbled up to the `cb_each()` call
site. In fact, this is a rather common pattern we have: whenever such a
callback function returns a non-zero error code, we abort iteration and
bubble up the code as-is.

Refactor both the crit-bit tree and oidtree subsystems to behave
accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>