doctor: Skip outdated check when offline

Fixes Homebrew/homebrew#13535.

Library/Homebrew/cmd/doctor.rb

@@ -856,8 +856,10 @@ def check_for_outdated_homebrew
   HOMEBREW_REPOSITORY.cd do
     if File.directory? ".git"
       local = `git rev-parse -q --verify refs/remotes/origin/master`.chomp
-      remote = /^([a-f0-9]{40})/.match(`git ls-remote origin refs/heads/master`)[0]
-      return if local == remote
+      remote = /^([a-f0-9]{40})/.match(`git ls-remote origin refs/heads/master 2>/dev/null`)
+      if remote.nil? || local == remote[0]
+        return
+      end
     end
 
     timestamp = if File.directory? ".git"