merge should include pending merges when computing merge base

[Martin Pool]

If there are already pending merges in a working tree, and we do a further merge into the same tree, we should include the pending merges as part of choosing the merge base etc. At the moment we correctly append to the pending parent list, but we do the merge just against the primary parent.

This might be fairly easy to change, if it's just a matter of passing more parents in to the merge code. If any of them assume there are only two revisions being merged it may be harder.

[Stefan Monnier (monnier)]

A possibly related issue is that "bzr merge --force -r A..B" will add the A..B to the pending merges (i.e. will keep track of history) if A is already present in the branch, but not if A is only present in the pending merges.