packaging: modify rc detection to work with X.Yrc instead of X.Y-rc
rc detection on CentOS failed without this change,
resulting in upgrades from 4.9rc to 4.9 not working
(4.9rc was considered more recent than 4.9).
Differential Revision: https://phab.mercurial-scm.org/D5876
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).