Replace grep in filter_ignore_files() with git ls-files --ignored

[GlassGruber]

This is a follow up of #591

[mkllnk]

Thank you for this contribution. I didn't know about this option and it definitely seems to be the way to go. But I also think that we can now simplify this method a lot. The output of ls-files is a list of file names, not patterns. So we can use that list with easy comparison instead of using glob patterns. Unfortunately, I don't have time to do this myself. Would you like to get more into bash coding?

[GlassGruber]

Hei there, yea I'm glad to contribute and hopefully I'll not break anything!

Took me a bit to grok the slice and dice in the filtering functions with the NUL separator bit, but eventually I remembered I did something similar before with rip-grep and adapted to grep.

My coding can be a bit verbose with comments and airy with spaces so if you prefer more terseness please edit as you like.

With a couple of very simple tests everything looks like working as expected.
I don't know the potential performance implications of grep in the filtering, I guess nothing really concerning, maybe in case of a very huge commit or similar? Anyway I haven't noticed anything slower in my few tests.