reachable: use traverse_commit_list instead of custom walk

reachable: use traverse_commit_list instead of custom walk

To find the set of reachable objects, we add a bunch of
possible sources to our rev_info, call prepare_revision_walk,
and then launch into a custom walker that handles each
object top. This is a subset of what traverse_commit_list
does, so we can just reuse that code (it can also handle
more complex cases like UNINTERESTING commits and pathspecs,
but we don't use those features).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>