CONTRIBUTING

fancyopts: use getopt.gnu_getopt()

The issue described in the docstring has been fixed since Python 20ab2260dc93,
which is in 2.7.

https://hg.python.org/cpython/rev/20ab2260dc93
https://bugs.python.org/issue4458

This fixes the handling of '--' value.

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).