tests/test-issue1089

tests: fix bashism to load helpers.sh

`source` isn't available at POSIX sh.

#!/bin/sh

hg init a
cd a
mkdir a
echo a > a/b
hg ci -Am m
hg rm a
hg ci -m m a

mkdir a b
echo a > a/b
hg ci -Am m
hg rm a
cd b
# relative delete
hg ci -m m ../a