Code cells with just one bash command are converted to markdown

[mwouts]

While testing jupytext==1.0.0-rc5 on Jake Vanderplas' notebooks for data-science (see also #99) with commands

git clone https://github.com/jakevdp/PythonDataScienceHandbook.git
cd PythonDataScienceHandbook/notebooks

# Update nbformat
jupytext --from ipynb --to ipynb *.ipynb
git commit -am "nbformat version"

# Test command
jupytext --to py:light --test *.ipynb

# Convert to a specific format, update
jupytext --to python/light//py:light *.ipynb
jupytext --from python/light//py:light --to ipynb --update python/light/*

# Show diffs
git diff

we observe that

• --quiet instruction in --test mode removes all output - it should still show the differences
• cells with only one magic instruction like %load_ext line_profiler gain a lines_to_next_cell cell metadata (both in light and percent formats)
• cells with just one bash instruction like !head -4 data/president_heights.csv are incorrectly converted from code to markdown (only in the light format)