`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.
ClosesHomebrew/homebrew#25230.
Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
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