attr_file: refactor stripping of trailing spaces The stripping of trailing spaces currently happens as part of `git_attr_fnmatch__parse`. As we aren't currently parsing trailing whitespaces correct in case they're escaped, we'll have to change that code, though. To make actual behavioural change easier to review, refactor the code up-front by pulling it out into its own function that is expected to retain the exact same functionality as before. Like this, the fix will be trivial to apply.