[merge] Make merge internals private

[Aaron Bentley]

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Could I get a review please?

Same old place:
http://panoramicfeedback.com/opensource/bzr/bzr.ab/

Martin Pool wrote:

>>>Perhaps if this is meant to
>>>be a bzrlib-internal class it would be better called
>>>_MergeConflictHandler.  (In which case it could go back in merge.py if
>>>you wanted :-)

This is done.

I have moved merge.merge into builtins so that we don't open branches
outside the UI level, as John Meinel suggested.

I have reduced the PEP8 violations.

get_tree and get_revid_tree are now private.

I have removed file_exists.

I have not removed build_working_dir, but I have explained why I think
it should not be removed.

I have not made merge_inner or Merge private, but I have explained why I
think they should be public.

I have not made MergeConflictHandler public, but I have explained why I
think it should not be public.

Aaron
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