fix broken environment save/restore when a hook runs. move "run commend with different env/cwd" code out to function in util. new function is called esystem.

#!/bin/sh

hg init
echo This is file a1 > a
hg add a
hg commit -m "commit #0" -d "0 0"
touch b
hg add b
rm b
hg commit -A -m"comment #1" -d "0 0"