config: make config.items() return a copy config.items() was iterating over a copy of the data for the the specified section on Python 2 by using .items(). However, on Python 3, items() does not make a copy, so let's switch to explicitly making a copy to make it safe on both Python 2 and Python 3.

filemerge: store error messages in module variables Copytracing may be disabled because it's too slow (see experimental.disablecopytrace config option). In that case user may get errors like 'local changed FILE which other deleted'. It would be nice to give user a hint to rerun command with `--config experimental.disablecopytrace=False`. To make it possible let's extract error message to variables so that extension may overwrite them.