wix: restore COPYING.rtf This got truncated to 0 bytes in 0ab651b5f77c when the Phabricator extension crashed because it's a binary file. That caused the license page in the WIX installer to be empty. I don't remember if I needed to resubmit after the bug was fixed, so let's try this again with the current stable. If this fails, I'll retry with 5.1 to see if this is a regression in the API changeover last cycle. Differential Revision: https://phab.mercurial-scm.org/D8052