Add utf8_fprintf helper that returns correct number of columns

Add utf8_fprintf helper that returns correct number of columns

Since command usages can be translated, they may include utf-8
encoded strings, and the output in console may not align well any
more. This is because strlen() is different from strwidth() on utf-8
strings.

A wrapper utf8_fprintf() can help to return the correct number of
columns required.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>