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
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