git: fix parse_git_dirty()

If oh-my-zsh.hide-status is configured, the 'clean' code won't be
generated, and some themes might end up distorted. Let's generate the
'clean' code even when we don't want the show the dirty status.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Merge pull request #1740 from felipec/fc/git

git: fix parse_git_dirty()