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
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