Encode ' ' using '=20' even though rfc2047 allows using '_' for
readability. Unfortunately, many programs do not understand this and
just leave the underscore in place. Using '=20' seems to work better.
[jc: with adjustment to t3901]
Signed-off-by: Kristian Høgsberg <hoegsberg@gmail.com> Signed-off-by: Junio C Hamano <junkio@cox.net>