findrenames: speedup exact match

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 28.890 secs (user 26.920+0.000 sys 1.450+0.000)
after : Time: real 6.790 secs (user 5.310+0.000 sys 1.350+0.000)

mercurial/cmdutil.py

@@ -276,11 +276,15 b' def findrenames(repo, added, removed, th'
         if r not in ctx:
             continue
         fctx = ctx.filectx(r)
-        orig = fctx.data()
 
         def score(text):
             if not len(text):
                 return 0.0
+            if not fctx.cmp(text):
+                return 1.0
+            if threshold == 1.0:
+                return 0.0
+            orig = fctx.data()
             # bdiff.blocks() returns blocks of matching lines
             # count the number of bytes in each
             equal = 0