add merge command. means same thing as "update -m". repo.addchangegroup method now returns number of heads modified and added, so command line can tell whether update or merge needed. this makes tiny change to ssh wire protocol, but change is backwards compatible. pull command now returns 0 if no changes to pull.

%%% should show a removed and b added
A b
R a
reverting...
%%% should show b unknown and a back to normal
? b
merging a
%%% should show foo-b
foo-b
%%% should show a removed and b added
A b
R a
reverting...
%%% should show b unknown and a marked modified (merged)
? b
%%% should show foo-b
foo-b