diff against explicit revision; fixes #15

Command 'bzr status' produces the message,
"working tree is out of date, run 'bzr update'"
after the go tool runs 'bzr update -r go1'.
It seems to be comparing the working tree against
a revision other than the one currently checked out.
Running diff against the exact revision produced by
IdentifyCmd is what we mean: whether the current
work tree is exactly the same as that commit.