By Tom Hals WILMINGTON, Del. (Reuters) -Elon Musk countersued Twitter Inc on Friday, escalating his legal fight against the social media company over his bid to walk away from the $44 billion purchase, although the lawsuit was filed confidentially. Although the 164-page document was not publicly available, under court rules a redacted version could soon be made public. Musk’s lawsuit was filed hours after Chancellor Kathaleen McCormick of the Delaware Court of Chancery ordered a five-day trial beginning Oct. 17 to determine if Musk can walk away from the deal. Twitter did not immediately respo…