discovery: abort also when pushing multiple headed new branch
Before this patch, pushing with --new-branch permits to create
multiple headed branch on the destination repository.
But permitting to create new branch should be different from
permitting to create multiple heads on branch.
This patch prevents from careless pushing multiple headed new branch,
and requires --force to push such branch forcibly.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.