i18n: make the translation of -u advice in one go

The advice (consider use of -u when read_directory takes too long) is
separated into 3 different status_printf_ln() calls, and which brings
trouble for translators.

Since status_vprintf() called by status_printf_ln() can handle eol in
buffer, we could simply join these lines into one paragraph.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>