t4055 failure: see error

Another problem caused by BSD v GNU sed, I think.

$ ./t4055-diff-context.sh -i -v
Initialized empty Git repository in /Users/brian/dev/git/t/trash directory.t4055-diff-context/.git/
expecting success: 
	cat >x <<-\EOF &&
	firstline
	b
	c
	d
	e
	f
	preline
	postline
	i
	j
	k
	l
	m
	n
	EOF
	git update-index --add x &&
	git commit -m initial &&

	git cat-file blob HEAD:x |
	sed "/preline/a\
	ADDED" >x &&
	git update-index --add x &&
	git commit -m next &&

	git cat-file blob HEAD:x |
	sed s/ADDED/MODIFIED/ >x

[master (root-commit) d5fa2e5] initial
 Author: A U Thor [off-list ref]
 1 file changed, 14 insertions(+)
 create mode 100644 x
sed: 1: "/preline/a	ADDED": command a expects \ followed by text