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.

removing b
this update spans a branch affecting the following files:
 b
aborting update spanning branches!
(use 'hg merge' to merge across branches or '-C' to lose changes)