copies: introduce getdstfctx Previously `c2` may had an incorrect linkrev because getsrcfctx set wrong _descendantrev. getsrcfctx() sets descendant rev equals to srcctx.rev() (see _makegetfctx()), but for `c2` descendant rev should be dstctx. While we were lucky it didn't broke copytracing it made it significantly slower in some cases. Besides it broke some external extensions, for example remotefilelog.