remote-bzr: fix partially pushed merge

remote-bzr: fix partially pushed merge

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