diagnostic: check homebrew-core git status.

Also look in Homebrew/homebrew-core for uncommitted files (in some ways
we care about uncommitted stuff there more than Homebrew/brew). Also,
list all the modified files even when on non-CI to make it easier for
users to figure out their own issues and check the contents of the
whole repos rather than just a subdirectory.

In general this is a cleanup that should have been done long ago (when
we split Homebrew/brew and Homebrew/homebrew-core).

Library/Homebrew/diagnostic.rb

@@ -643,26 +643,31 @@ module Homebrew
       def check_git_status
         return unless Utils.git_available?
 
-        modified = []
-        HOMEBREW_REPOSITORY.cd do
-          status = `git status --untracked-files=all --porcelain -- Library/Homebrew/ 2>/dev/null`
-          return if status.blank?
+        message = nil
 
-          modified = status.split("\n")
+        {
+          "Homebrew/brew"          => HOMEBREW_REPOSITORY,
+          "Homebrew/homebrew-core" => CoreTap.instance.path,
+        }.each do |name, path|
+          status = path.cd do
+            `git status --untracked-files=all --porcelain 2>/dev/null`
           end
+          next if status.blank?
 
-        message = <<~EOS
-          You have uncommitted modifications to Homebrew.
+          message ||= ""
+          message += "\n" unless message.empty?
+          message += <<~EOS
+            You have uncommitted modifications to #{name}.
             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 #{HOMEBREW_LIBRARY} && git stash && git clean -d -f
+              cd #{path} && git stash && git clean -d -f
           EOS
 
-        if ENV["CI"]
+          modified = status.split("\n")
           message += inject_file_list modified, <<~EOS
 
-            Modified files:
+            Uncommitted files:
           EOS
         end