Use git's -C option to set git's working directory for uncommitted modifications nudge

Library/Homebrew/diagnostic.rb

@@ -703,7 +703,7 @@ module Homebrew
             If this is a surprise to you, then you should stash these modifications.
             Stashing returns Homebrew to a pristine state but can be undone
             should you later need to do so for some reason.
-              pushd #{path} && git stash -u && git clean -d -f && popd
+              git stash -u -C #{path} && git clean -d -f -C #{path}
           EOS
 
           modified = status.split("\n")