tests/test-issue612

replace usage of os.popen() with util.popen()

To make this possible, I added a mode parameter to both implementations of
util.popen(), defaulting to 'r' (as it does in the Python stdlib).

#!/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