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 <author@example.com>
1 file changed, 14 insertions(+)
create mode 100644 x
sed: 1: "/preline/a ADDED": command a expects \ followed by text