rust-core: updated copyright notice

The intention here is to put an email address that actually
works (I did have full personal copyrights with my
former affiliation).

It is also an opportunity to acknowledge that I've not been
the only one to work on this file.

Differential Revision: https://phab.mercurial-scm.org/D7781

rust/hg-core/src/lib.rs

@@ -1,4 +1,5 b''
-// Copyright 2018 Georges Racinet <[e-mail]>
+// Copyright 2018-2020 Georges Racinet <[e-mail]>
+//           and Mercurial contributors
 //
 // This software may be used and distributed according to the terms of the
 // GNU General Public License version 2 or any later version.