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.

merge with crew.

merge with crew.

fix backtrace printed when cannot get lock. change lock error handling code so exceptions have useful info and exception handling in one place. add test case for when cannot get lock.