convert: always initialize attr_action in convert_attrs

convert: always initialize attr_action in convert_attrs

convert_attrs contains an "if-else". In the "if", we set attr_action
twice, and the first assignment has no effect. In the "else", we do not
set it at all. Since git_check_attr always returns the same value, we'll
always end up in the "if", so there is no problem right now. But
convert_attrs is obviously trying not to rely on such an
implementation-detail of another component.

Make the initialization of attr_action after the if-else. Remove the
earlier assignments.

Suggested-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>