revwalk: remove a useless enqueueing phase for topological and default sorting
revwalk: remove a useless enqueueing phase for topological and default sorting

After `limit_list()` we already have the list in time-sorted order, which is
what we want in the "default" case. Enqueueing into the "unsorted" list would
just reverse it, and the topological sort will do its own sorting if it needs
to.

stewid/libgit2,stewid/libgit2,stewid/libgit2,Corillian/libgit2,Corillian/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,Corillian/libgit2,Corillian/libgit2,Corillian/libgit2,Corillian/libgit2