findrenames: first loop over the removed files, it's faster

Getting the file from the working dir is less expensive than getting it from
the repo history, hence the speedup.

benchmarked on crew repo with:
rm -rf * ; hg up -C ; for i in `find . -name "*.py"` ; do mv $i $i.new;done

followed by:
hg addremove -s 100

before: Time: real 68.760 secs (user 65.760+0.000 sys 2.490+0.000)
after : Time: real 28.890 secs (user 26.920+0.000 sys 1.450+0.000)

mercurial/cmdutil.py

@@ -270,15 +270,16 b' def matchfiles(repo, files):'
 
 def findrenames(repo, added, removed, threshold):
     '''find renamed files -- yields (before, after, score) tuples'''
+    copies = {}
     ctx = repo['.']
-    for a in added:
-        aa = repo.wread(a)
-        bestname, bestscore = None, threshold
-        for r in removed:
-            if r not in ctx:
-                continue
-            rr = ctx.filectx(r).data()
-
+    for r in removed:
+        if r not in ctx:
+            continue
+        fctx = ctx.filectx(r)
+        rr = fctx.data()
+        for a in added:
+            bestscore = copies.get(a, (None, threshold))[1]
+            aa = repo.wread(a)
             # bdiff.blocks() returns blocks of matching lines
             # count the number of bytes in each
             equal = 0
@@ -292,9 +293,10 b' def findrenames(repo, added, removed, th'
             if lengths:
                 myscore = equal*2.0 / lengths
                 if myscore >= bestscore:
-                    bestname, bestscore = r, myscore
-        if bestname:
-            yield bestname, a, bestscore
+                    copies[a] = (r, myscore)
+    for dest, v in copies.iteritems():
+        source, score = v
+        yield source, dest, score
 
 def addremove(repo, pats=[], opts={}, dry_run=None, similarity=None):
     if dry_run is None: