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.

%% no changes
%% no changes
%% uncompressed contents in repo
this is a test
%% uncompress our working dir copy
this is a test
%% uncompress our new working dir copy
this is a test