Find the system's MD5 binary. Different binaries calculate MD5 checksums on different systems. Try a couple known programs and only calculate and verify the checksum if they exist. This should silence warnings on eg OpenBSD.

#!/bin/sh

mkdir a
cd a
hg init
echo 123 > a
hg add a
hg commit -m "a" -u a -d "1000000 0"

cd ..
mkdir b
cd b
hg init
echo 321 > b
hg add b
hg commit -m "b" -u b -d "1000000 0"

hg pull ../a
hg pull -f ../a
hg heads