Emit warning when no editor is set

Closes Homebrew/homebrew#34647.

Library/Homebrew/utils.rb

@@ -208,17 +208,24 @@ end
 
 def which_editor
   editor = ENV.values_at('HOMEBREW_EDITOR', 'VISUAL', 'EDITOR').compact.first
-  # If an editor wasn't set, try to pick a sane default
   return editor unless editor.nil?
 
   # Find Textmate
-  return 'mate' if which "mate"
+  editor = "mate" if which "mate"
   # Find BBEdit / TextWrangler
-  return 'edit' if which "edit"
+  editor ||= "edit" if which "edit"
   # Find vim
-  return 'vim' if which "vim"
+  editor ||= "vim" if which "vim"
   # Default to standard vim
-  return '/usr/bin/vim'
+  editor ||= "/usr/bin/vim"
+
+  opoo <<-EOS.undent
+    Using #{editor} because no editor was set in the environment.
+    This may change in the future, so we recommend setting EDITOR, VISUAL,
+    or HOMEBREW_EDITOR to your preferred text editor.
+  EOS
+
+  editor
 end
 
 def exec_editor *args