Factor is_potential_path to properly handle CDPATH

This will let us color cd commands better

path.h

@@ -68,6 +68,7 @@ bool path_is_valid(const wcstring &path, const wcstring &working_directory);
 /** Returns whether the two paths refer to the same file */
 bool paths_are_same_file(const wcstring &path1, const wcstring &path2);
 
+/* Returns the current working directory as returned by wgetcwd */
 wcstring get_working_directory(void);
 
 #endif