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

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m "test" -d "0 0"
hg verify
hg parents
hg status
hg undo
hg verify
hg parents
hg status