copies: avoid materializing a full directory map during copy tracing

Materializing a full copy of every directory in a treemanifest repo can be quite
expensive, even with a narrow matcher. For flat manifest repos, this should be
equivalent - it will still materialize (and cache) a dict of all of the dirs
inside of the manifest object, we just don't get a copy of it.

In a repo I have here, this brings the time for a simple rebase from 11.197s to
4.609s.

Differential Revision: https://phab.mercurial-scm.org/D9503

mercurial/copies.py

@@ -930,7 +930,6 b' def _dir_renames(repo, ctx, copy, fullco'
     addedfiles: added files on the other side (compared to ctx)
     """
     # generate a directory move map
-    d = ctx.dirs()
     invalid = set()
     dirmove = {}
 
@@ -941,7 +940,7 b' def _dir_renames(repo, ctx, copy, fullco'
         if dsrc in invalid:
             # already seen to be uninteresting
             continue
-        elif dsrc in d and ddst in d:
+        elif ctx.hasdir(dsrc) and ctx.hasdir(ddst):
             # directory wasn't entirely moved locally
             invalid.add(dsrc)
         elif dsrc in dirmove and dirmove[dsrc] != ddst:
@@ -954,7 +953,7 b' def _dir_renames(repo, ctx, copy, fullco'
     for i in invalid:
         if i in dirmove:
             del dirmove[i]
-    del d, invalid
+    del invalid
 
     if not dirmove:
         return {}, {}