hgweb: display diff for a changeset against any parents (issue2810) During merge of branches, it is useful to compare merge results against the two parents. This change adds this support to hgweb. To specify which parent to compare to, use rev/12300:12345 where 12300 is a parent changeset number. Two links are added to changeset web page so that one can choose which parent to compare to.

To run the tests, do:

cd tests/
python run-tests.py

See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.