strbuf: convenience format functions with \n automatically appended

strbuf: convenience format functions with \n automatically appended

These functions are helpful when we do not want to expose \n to
translators. For example

    printf("hello world\n");

can be converted to

    printf_ln(_("hello world"));

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>