copies: write empty entries in changeset when also writing to filelog
When writing to both changeset and filelog (during transition), we
don't want the reader to waste time by falling back to reading from
the filelog when there is no copy metadata. Let's write out empty copy
metadata instead (the read path is already prepared for this
case). Thanks to Greg for pointing this out.
Differential Revision: https://phab.mercurial-scm.org/D6306