doctor: don't let gitconfig break dirty-tree check

`brew doctor` always complains about uncommitted modifications to
Homebrew when you have status.branch=true in your git config, because
the implicit --branch makes `git status -s` always print branch/tracking
info. The --porcelain mode is similar to -s/--short, but ignores most of
the user's config and should remain stable.

The --untracked-files option ensures we'll see untracked files even for
users with status.showUntrackedFiles=no, which is not suppressed by
--porcelain for some reason.