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
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed