If part of the merge was already pushed, we don't have the blob_marks
available, however, the commits are already stored in bazaar, so we can
use the revision_tree to fetch the contents.
We want to do this only when there's no other option.
There's no easy way to test this.
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>