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.

mercurial/copies.py

@@ -632,6 +632,7 b' def _checkcopies(srcctx, dstctx, f, base'
     # We use 'backwards = False' in that case.
     backwards = not remotebase and base != tca and f in mb
     getsrcfctx = _makegetfctx(srcctx)
+    getdstfctx = _makegetfctx(dstctx)
 
     if msrc[f] == mb.get(f) and not remotebase:
         # Nothing to merge
@@ -658,7 +659,7 b' def _checkcopies(srcctx, dstctx, f, base'
             continue # no match, keep looking
         if mdst[of] == mb.get(of):
             return # no merge needed, quit early
-        c2 = getsrcfctx(of, mdst[of])
+        c2 = getdstfctx(of, mdst[of])
         # c2 might be a plain new file on added on destination side that is
         # unrelated to the droids we are looking for.
         cr = _related(oc, c2, tca.rev())