Enable "hg push --new-branch" by default in hgrc

[Dominik Psenner]

I've got another usecase for "issue" named branches. A bugtracking tool
could easily gather changesets related to the issue identified by IssueId
and link to those changes by invoking something like:

$ hg log -b IssueId

Would you please tell me more about possible scalability problems and
(computational) borders that can be encountered?