Fix _git_dirty so that it works with git 1.8.1.1

Since 1.8.1.1 [1] "git status" on a clean tree displays
"nothing to commit, working directory clean" instead of
"nothing to commit, (working directory clean)"

[1] https://github.com/git/git/commit/50bd8b7eb960ff7a646195e0751b48a0e57e03ad#L0R1050