pull: fix post-pull common computation
Before that changes, pulled revision that happend to be already known locally
(so, not actually added) was not taken into account when computing the new
common set between local and remote.
It appears that we already know the heads of the pulled set. It is in the
`rheads` variable, so we are just using it and everything is works fine.
We are dropping the, now useless, computation of `added` set in the process.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.