Marking file as non-binary

For whatever reason, git claims that my foo.cpp (c++ source) is binary,
which means I see no diff when doing git log -p. I already tried putting

*.cpp diff=true text=true

in .gitattributes in maybe 20 different variations, but it doesn't help
at all. I've checked the filename for typos, etc. Today is a day when
really nothing works here.