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.

Closes Homebrew/homebrew#25230.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/doctor.rb

@@ -909,7 +909,7 @@ end
 def check_git_status
   return unless which "git"
   HOMEBREW_REPOSITORY.cd do
-    unless `git status -s -- Library/Homebrew/ 2>/dev/null`.chomp.empty?
+    unless `git status --untracked-files=all --porcelain -- Library/Homebrew/ 2>/dev/null`.chomp.empty?
       <<-EOS.undent_________________________________________________________72
       You have uncommitted modifications to Homebrew
       If this a surprise to you, then you should stash these modifications.