Pygments assumes that .txt files should be styled using ResourceLexer by
default, when guessing by filename. This is very often wrong, and
results in a bunch of error indicators in the style, for practically
every word in the file.

We now maintain a blacklist of filename extensions that won't be styled.
It so far just contains .txt files.

Loaded a text file in the diff viewer before the fix, and saw all the

error indicators.
Loaded a text file after the fix and saw that it looked like a plain

text file.