tests/test-diff-binary-file

tests: fix bashism to load helpers.sh

`source` isn't available at POSIX sh.

#!/bin/sh

hg init a
cd a
cp $TESTDIR/binfile.bin .
hg add binfile.bin
hg ci -m 'add binfile.bin'

echo >> binfile.bin
hg ci -m 'change binfile.bin'

hg revert -r 0 binfile.bin
hg ci -m 'revert binfile.bin'

echo % diff -r 0 -r 1
hg diff --nodates -r 0 -r 1

echo % diff -r 0 -r 2
hg diff --nodates -r 0 -r 2

echo % diff --git -r 0 -r 1
hg diff --git -r 0 -r 1

echo % diff --git -r 0 -r 2
hg diff --git -r 0 -r 2