diagnostic: Use pushd/popd in uncommited modifications nudge

While filing #20379, I lost where I was because I followed the given instruction. Instead, let's use pushd/popd so the user is returned to their pwd if the git operations succeed.

Author: colindean

Library/Homebrew/diagnostic.rb

@@ -703,7 +703,7 @@ def check_git_status
             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.
-              cd #{path} && git stash -u && git clean -d -f
+              pushd #{path} && git stash -u && git clean -d -f && popd
           EOS
 
           modified = status.split("\n")