When a path is unmerged in the index, we used to always say "unmerged" in
the "Changed but not updated" section, even when the path was deleted in
the work tree.

Remove unmerged entries from the "Updated" section, and create a new
section "Unmerged paths".  Describe how the different stages conflict
in more detail in this new section.

Note that with the current 3-way merge policy (with or without recursive),
certain combinations of index stages should never happen.  For example,
having only stage #2 means that a path that did not exist in the common
ancestor was added by us while the other branch did not do anything to it,
which would have autoresolved to take our addition.  The code nevertheless
prepares for the possibility that future merge policies may leave a path
in such a state.

Signed-off-by: Junio C Hamano <gitster@pobox.com>