copies._forwardcopies: use set operations to find missing files

This is a performance win for a number of reasons:
- We don't iterate over contexts, which avoids a completely unnecessary sorted
  call + the O(number of files) abstraction cost of doing that.
- We don't check membership in a context, which avoids another
  O(number of files) abstraction cost.
- We iterate over the manifests in C instead of Python.

For a large repo with 170,000 files, this improves perfpathcopies from 0.34
seconds to 0.07. Anything that uses pathcopies, such as rebase or diff --git
between two revisions, benefits.

mercurial/copies.py

@@ -133,8 +133,10 b' def _forwardcopies(a, b):'
     # we currently don't try to find where old files went, too expensive
     # this means we can miss a case like 'hg rm b; hg cp a b'
     cm = {}
-    for f in b:
-        if f not in a:
+    missing = set(b.manifest().iterkeys())
+    missing.difference_update(a.manifest().iterkeys())
+
+    for f in missing:
             ofctx = _tracefile(b[f], a)
             if ofctx:
                 cm[f] = ofctx.path()