tests/test-issue612

tests: fix bashism to load helpers.sh

`source` isn't available at POSIX sh.

#!/bin/sh

mkdir t
cd t

hg init
mkdir src
echo a > src/a.c
hg ci -Ama -d "10000000 0"

hg mv src source
hg ci -Ammove -d "1000000 0"

hg co -C 0
echo new > src/a.c
echo compiled > src/a.o
hg ci -mupdate -d "1000000 0"

hg st

hg merge

hg st