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.

adding foo
adding bomb
adding a.c
adding dir/a.o
adding dir/b.o
! a.c
! dir/a.o
! dir/b.o
? .hgignore
a.c: unsupported file type (type is fifo)
! a.c