Allow exclude_files option in the config file to exclude a specific file in a subdirectory #254

Currently, this comment from 2014 in the ProcessConfigOverrides function says:

if we are checking for lint errors in /foo/bar/baz.cc and we found the .cfg file at /foo/CPPLINT.cfg, then the config file's "exclude_files" filter is meant to be checked against "bar" and not "baz" nor "bar/baz.cc".

This undocumented behavior makes no sense to me and would probably confuse people. Opening this issue to see if anyone'd oppose unlimiting this behavior.