copies: do full filtering at end of _changesetforwardcopies()

As mentioned earlier, pathcopies() is very slow when copies are stored
in the changeset. Most of the cost comes from calling _chain() for
every changeset, which is slow because it needs to read manifests. It
needs to read manifests to be able to filter out copies that are were
created in one commit and then deleted. (It also filters out copies
that were created from a file that didn't exist in the starting
revision, but that's a fixed revision across calls to _chain(), so
it's much cheaper.)

This patch changes from _chainandfilter() to just _chain() in the main
loop in _changesetforwardcopies(). It instead removes copies that have
subsequently been removed by using ctx.filesremoved(). We thus rely on
that to be fast.

It timed this command in mozilla-unified:

  hg debugpathcopies FIREFOX_59_0b3_BUILD2 FIREFOX_BETA_59_END

It took 18s before and 1.1s after. It's still faster when copy
information is stored in filelogs: 0.70s. It also still gets slow when
there are merge commits involved, because we read manifests there
too. We'll deal with that later.

Differential Revision: https://phab.mercurial-scm.org/D6419

mercurial/copies.py

@@ -300,6 +300,7 b' def _changesetforwardcopies(a, b, match)'
         else:
             copies = copies1
         if r == b.rev():
+            _filter(a, b, copies)
             return copies
         for c in children[r]:
             childctx = repo[c]
@@ -313,7 +314,10 b' def _changesetforwardcopies(a, b, match)'
             if not match.always():
                 childcopies = {dst: src for dst, src in childcopies.items()
                                if match(dst)}
-            childcopies = _chainandfilter(a, childctx, copies, childcopies)
+            childcopies = _chain(copies, childcopies)
+            for f in childctx.filesremoved():
+                if f in childcopies:
+                    del childcopies[f]
             heapq.heappush(work, (c, parent, childcopies))
     assert False