Add a new test that documents how to use the pre-commit hook with '--set-formats'

[mwouts]

The test works without filtering on .ipynb files thanks to the resolution of #969 that has been integrated in this branch.

Closes #967 .

[codecov]

Codecov Report

Merging #971 (e0be3bd) into main (4507f81) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #971      +/-   ##
==========================================
+ Coverage   99.03%   99.07%   +0.04%     
==========================================
  Files         117      118       +1     
  Lines       10898    10946      +48     
==========================================
+ Hits        10793    10845      +52     
+ Misses        105      101       -4     

Impacted Files	Coverage Δ
tests/conftest.py	100.00% <ø> (ø)
jupytext/cli.py	95.85% <100.00%> (-0.02%)	⬇️
tests/test_cli.py	100.00% <100.00%> (ø)
tests/test_pre_commit_1_sync_with_no_config.py	100.00% <100.00%> (ø)
jupytext/__main__.py	100.00% <0.00%> (+100.00%)	⬆️

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