tests/test-diff-unified

tests: fix bashism to load helpers.sh

`source` isn't available at POSIX sh.

#!/bin/sh

hg init repo
cd repo
cat > a <<EOF
c
c
a
a
b
a
a
c
c
EOF
hg ci -Am adda
cat > a <<EOF
c
c
a
a
dd
a
a
c
c
EOF

echo '% default context'
hg diff --nodates

echo '% invalid --unified'
hg diff --nodates -U foo

echo '% --unified=2'
hg diff --nodates -U 2

echo '% diff.unified=2'
hg --config diff.unified=2 diff --nodates

echo '% diff.unified=2 --unified=1'
hg diff --nodates -U 1

echo '% invalid diff.unified'
hg --config diff.unified=foo diff --nodates

echo % test off-by-one error with diff -p
hg init diffp
cd diffp
echo a > a
hg ci -Ama
rm a
echo b > a
echo a >> a
echo c >> a
hg diff -U0 -p --nodates

exit 0