When doing a git grep (of rust-analyzer or of rust-lang/rust with
--recurse-submodules), if the grep happens to match within the large
benchmark data files, the resulting long single lines can cause a text
pager or editor to slow down and distract from more useful matches.
These test data files aren't formatted for human consumption, so mark
them as binary, which causes git grep to instead just state that they
match without printing the matching "line".
