`black.reformat_many` depends on a lot of slow-to-import modules. When
formatting simply a single file, the time paid to import those modules
is totally wasted. So I moved `black.reformat_many` and its helpers
to `black.concurrency` which is now *only* imported if there's more
than one file to reformat. This way, running Black over a single file
is snappier
Here are the numbers before and after this patch running `python -m
black --version`:
- interpreted: 411 ms +- 9 ms -> 342 ms +- 7 ms: 1.20x faster
- compiled: 365 ms +- 15 ms -> 304 ms +- 7 ms: 1.20x faster
Co-authored-by: Fabio Zadrozny <fabiofz@gmail.com>
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