Fail demo action for untracked files

[FriesischScott]

The current action will only fail if demo files are modified after running Literate.jl. This way the action will also fail for new untracked files.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e2d4772) 98.27% compared to head (82e9952) 98.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #157 +/- ## ======================================= Coverage 98.27% 98.27% ======================================= Files 27 27 Lines 1045 1045 ======================================= Hits 1027 1027 Misses 18 18 ```

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