[MERGE] new merge-type=lca

[Andrew Bennetts]

Aaron Bentley wrote:
> Hi all,
> 
> This patch implements support for the LCA merge type, which I believe is
> a great general-purpose merge algorithm, with excellent criss-cross
> support.  I'm very excited about it, so grab a coffee, and I'll yabber on.

I just happened to do a criss-cross merge that was producing conflicts, so I
tried this out.  This is pretty unscientific, but here's what happened:

  * bzr.dev default (merge3): 4 conflicts, took ~6.5s.
  * weave: too slow.  I gave up waiting after a couple of minutes of 100% CPU (I
    presume the lack of annotation caches in my pack repository caused this).
  * lca: 1 conflict, took ~8.5s.

The conflicts were all pretty trivial anyway, but as a mere human it'd take more
than 2.1s for me to resolve them, so I'm very happy with --merge-type=lca so
far.  Good work!

-Andrew.