issue6528: add a config option to control the fixing on the fly This will allow people who know to be safe to avoid any performance overhead (and other potential issue). Differential Revision: https://phab.mercurial-scm.org/D11271

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