[MERGE] support merge when both branches have no reachable 'import' revisions

[Robert Collins]

This patch allows merge to find a base when there are no revisions
present in the repository with no parents : that is, all the 'import'
revisions are ghosts.

This is a not-uncommon situation with baz imports.

Cheers
Rob

-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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