Merging
=======

There should be one merge command which does the right thing, and
which is called 'merge'.

The merge command pulls a changeset or a range of changesets into your
tree.  It knows what changes have already been integrated and avoids
pulling them again.

There should be some intelligence about working out what changes have
already been merged. 

The tool intelligently chooses (or perhaps synthesizes) an ancestor
and two trees to merge.  These are then intelligently merged.

Merge should refuse to run (unless forced) if there are any
uncommitted changes in your tree beforehand.  This has two purposes:
if you mess up the merge you won't lose anything important; secondly
this makes it more likely that the merge will be relatively pure.