rust-status: explicitly track bad file types
Before this, we silently skipped bad file types.
Now, we check to see if a path is an exact_match in our matcher, and
if so, print an error for it, and only then do we skip it.
A path will be an exact match when it's specified as an explicit
command line argument, and this error-printing behavior is necessary
for compatibility with Python status.
# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable