config: use copy-on-write to improve copy performance Previously, chg's `verify` call could take 30+ms loading and checking new config files. With one socket redirection, that adds up to around 70ms, which is a lot for fast commands (ex. `bookmark --hidden`). When investigating closer, A lot of time was spent on actually spent on ui copying, which is mainly about `config.config` and `dict` copying. This patch makes that 20x faster by adopting copy-on-write. The copy-on-write is performed at config section level. Before: In [1]: %timeit ui.copy() 100 loops, best of 3: 2.32 ms per loop After: In [1]: %timeit ui.copy() 10000 loops, best of 3: 128 us per loop 2ms may look not that bad, but it adds up pretty quickly with multiple calls. A typical chg run may call it 4 times, which is about 10ms. Differential Revision: https://phab.mercurial-scm.org/D808