wix: restore COPYING.rtf
8427fea04017 accidentally blew away the content of this file.
As part of restoring the content, I updated the copyright year
to 2019.
Differential Revision: https://phab.mercurial-scm.org/D6098
store: move logic to check for invalid entry in fncache to own function
This helps separate the original reading logic from the one which finds for an
invalid entry.
Differential Revision: https://phab.mercurial-scm.org/D6030