builtin/merge-recursive: fix leaking object ID bases

builtin/merge-recursive: fix leaking object ID bases

In `cmd_merge_recursive()` we have a static array of object ID bases
that we pass to `merge_recursive_generic()`. This interface is somewhat
weird though because the latter function accepts a pointer to a pointer
of object IDs, which requires us to allocate the object IDs on the heap.
And as we never free those object IDs, the end result is a leak.

While we can easily solve this leak by just freeing the respective
object IDs, the whole calling convention is somewhat weird. Instead,
refactor `merge_recursive_generic()` to accept a plain pointer to object
IDs so that we can avoid allocating them altogether.

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