cdiff —check-style only checks for leading tabs, not tabs in leading whitespace

See title: cdiff's description indicates that it warns against "spurious changes", the changes it warns against include trailing white space and leading tabs, but if somebody breaks a file's indentation by adding tabs *after* spaces (in order to increase the indentation level with crummy tools), check-style will not output any warning.

check-style should at least warn when indentation is or becomes mixed, ideally it should warn against any tab character added to a file.

The current behavior gives a false sense of security (user may think he is protected against inserting tabs)