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.
adding foo
adding bar
adding baz/bletch
test-archive-TIP/bar
test-archive-TIP/baz/bletch
test-archive-TIP/foo
test-archive-TIP/bar
test-archive-TIP/baz/bletch
test-archive-TIP/foo
Archive: archive.zip
testing: test-archive-TIP/bar OK
testing: test-archive-TIP/baz/bletch OK
testing: test-archive-TIP/foo OK
No errors detected in compressed data of archive.zip.