Solaris' sed was choking on the i\ commands used in
t4015-diff-whitespace as it couldn't parse the program properly.
Modify two uses of sed that worked in GNU sed but not Solaris'
(/usr/bin or /usr/xpg4/bin) to an equivalent form that is handled
properly by both.
Signed-off-by: Ben Walton <bdwalton@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>