Skip the pre-commit tests when the Jupytext folder is not a git repo

[mwouts]

Closes #814

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@b32c4b4). Click here to learn what that means.
The diff coverage is 97.62%.

@@            Coverage Diff            @@
##             master     #815   +/-   ##
=========================================
  Coverage          ?   99.13%           
=========================================
  Files             ?      106           
  Lines             ?    10262           
  Branches          ?        0           
=========================================
  Hits              ?    10173           
  Misses            ?       89           
  Partials          ?        0           

Impacted Files	Coverage Δ
jupytext/reraise.py	25.00% <25.00%> (ø)
jupytext/kernels.py	92.85% <92.85%> (ø)
jupytext/pep8.py	95.58% <95.58%> (ø)
jupytext/cli.py	96.10% <96.04%> (ø)
jupytext/pandoc.py	96.61% <96.61%> (ø)
jupytext/cell_reader.py	97.18% <96.97%> (ø)
jupytext/cell_to_text.py	97.80% <97.16%> (ø)
jupytext/myst.py	97.19% <97.19%> (ø)
jupytext/config.py	97.28% <97.28%> (ø)
jupytext/contentsmanager.py	97.85% <97.73%> (ø)
... and 17 more

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