policy: relax the default for in-place build
We're going to make the 'c' policy more strict, where no missing attribute
will be allowed. Since we want to run 'hg bisect' without rebuilding the C
extension modules, we'll need a looser policy for development environment.
The default for system installation isn't changed.
Note that the current 'c' policy is practically 'allow'-ish as we have lots
of adhoc fallbacks to pure functions.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).