fix: duplication of directives with nbdev_update #1530

[daphn3k]

fixes #1530

The actual fix is a couple of lines in sync.ipynb 's function _update_nb().
Below it, I have added two cells for comparing nbdev_update with and without the fix.
To be able to compare (as well as reproducing the bug) I had to add a new notebook (tests/update_directives.ipynb) and its exported py file (nbdev/test_update_directives.py).

This is my first pull request to this repo, so apologies if it doesn't fully conform. Feedback is welcome :)

[kurianbenoy-sarvam]

@daphn3k can you request Jeremy to review this PR?

[jph00]

Many thanks for this PR! :)

tbh, it's way too much code. My basic premise is that I hate code. So every line of it must be justifiable by showing how much values it's adding. There are a lot of lines here not earning their keep. ;)

Instead, try to have just the absolute minimum about of new code to implement your feature, and to show it working. Don't add new files -- I add files of code even more than lines of code!