This patch replaces experimental.disablecopytrace with experimental.copytrace.
Since the words does not means the same, the default value is also changed. Now
experimental.copytrace defaults to 'off'. The new value is not boolean value as
we will be now having two different algorithms (current one and heuristics one
to be imported from fbext) so we need this to be have more options than
booleans.
The old config option is not kept is completely replaced as that was under
experimental and we don't gurantee BC to experimental things.
.. bc::
The config option for copytrace `experimental.disablecopytrace` is now replaced with `experimental.copytrace` which defaults to `on`. If you need to turn off copytracing, add `[experimental] copytrace = off` to your config.
This could be kept as configbool so any falsy value could be
specified, but I don't have strong preference.