Speed regression since 19.10b0 on large collection #1581
Description
I'm seeing a nasty speed regression on a 1500-line file containing a long list of objects:
$ black --version
black, version 19.10b0
$ time black data-infra-common/qschema/registry/data_infra.py
All done! ✨ 🍰 ✨
1 file left unchanged.
real 0m1.642s
user 0m1.620s
sys 0m0.024s
$ ~/repos/black/.venv/bin/black --version
black, version 19.10b1.dev134+gb59a524
$ time ~/repos/black/.venv/bin/black data-infra-common/qschema/registry/data_infra.py
All done! ✨ 🍰 ✨
1 file left unchanged.
real 0m58.674s
user 0m58.048s
sys 0m0.168s