git-svn: optimize --branch and --branch-all-ref

By breaking the pipe read once we've seen a commit twice.

This should make -B/--branch-all-ref faster and usable on a
frequent basis.

We use topological order now for calling git-rev-list, and any
commit we've seen before should imply that all parents have been
seen (at least I hope that's the case for --topo-order).

Signed-off-by: Eric Wong <normalperson@yhbt.net>