mailinfo: feed only one line to handle_filter() for QP input

mailinfo: feed only one line to handle_filter() for QP input

The function is intended to be fed one logical line at a time to
inspect, but a QP encoded raw input line can have more than one
lines, just like BASE64 encoded one.

Quoting LF as =0A may be unusual but RFC2045 allows it.

The issue was noticed and fixed by Jay Soffian.  JC added a test
to protect the fix from regressing later.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>