Rewrite merge functionality into its own module.

Rewrite merge functionality into its own module.

Previously, Wizard had a gigantic merge routine inside of
the upgrade command, and interweaved automated resolution
before and after the merge took place.

This commit reorganizes the merge functionality into
the wizard.merge module, and also pushes as much pre-merge
resolution to before merge, so that we can generate more
meaningful rerere post-images.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>