Merge mea culpa

[Aaron Bentley]

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Hi all,

I am sorry for the current mess in the merge code.  I've fixed things up
in my current devel branches, and I expect those will enter the mainline
very soon.

With those fixes,
1. We will copy over all the ancestors we can, even those not in the
revision history, when branching, pulling and merging
2. When you commit a merge, you'll get a revision with two parents
3. The common ancestor picking can now pick ancestor revisions that
aren't in the branch history, taking advantage of revisions produced by
the steps above.

Some possibly-surprising changes:

The "Added %d texts", and "Added %d inventories." messages may appear
multiple times while branching/pulling/merging.  This is because when we
install revisions, they may have ancestors that we lack.   So we go back
and copy these new ancestors.  And so on, until we've got every ancestor
we can see.

The 'Failed to install' messages are not warnings, even.  They just note
the revisions that could not be installed, because they weren't present
in the other branch.  There are a few revisions in the mainline that
have multiple ancestors, but don't have the second parent in the
mainline storage.  This problem should become less common over time.

My current devel branch is here:
http://panoramicfeedback.com/opensource/bzr.24/

Aaron
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