odb: get rid of `the_repository` in `for_each()` functions

odb: get rid of `the_repository` in `for_each()` functions

There are a couple of iterator-style functions that execute a callback
for each instance of a given set, all of which currently depend on
`the_repository`. Refactor them to instead take an object database as
parameter so that we can get rid of this dependency.

Rename the functions accordingly.

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