perf: store reference to revlog._chunkraw in a local variable

perf: store reference to revlog._chunkraw in a local variable

To prepare for renaming revlog._chunkraw, we stuff a reference to this
metho in a local variable. This does 2 things. First, it moves the
attribute lookup outside of a loop, which more accurately measures
the time of the code being invoked. Second, it allows us to alias
to different methods depending on their presence (perf.py needs to
support running against old Mercurial versions).

Removing an attribute lookup from a tigh loop appears to shift the
numbers slightly with mozilla-central:

$ hg perfrevlogchunks -c

! read
! wall 0.354789 comb 0.340000 user 0.330000 sys 0.010000 (best of 28)
! wall 0.335932 comb 0.330000 user 0.290000 sys 0.040000 (best of 30)
! read w/ reused fd
! wall 0.342326 comb 0.340000 user 0.320000 sys 0.020000 (best of 29)
! wall 0.332857 comb 0.340000 user 0.290000 sys 0.050000 (best of 30)
! read batch
! wall 0.023623 comb 0.020000 user 0.000000 sys 0.020000 (best of 124)
! wall 0.023666 comb 0.020000 user 0.000000 sys 0.020000 (best of 125)
! read batch w/ reused fd
! wall 0.023828 comb 0.020000 user 0.000000 sys 0.020000 (best of 124)
! wall 0.023556 comb 0.020000 user 0.000000 sys 0.020000 (best of 126)

--- a/contrib/perf.py	Sat May 06 12:02:12 2017 -0700
+++ b/contrib/perf.py	Sat May 06 12:02:31 2017 -0700
@@ -887,6 +887,7 @@
     see ``perfrevlog`` and ``perfrevlogrevision``.
     """
     rl = cmdutil.openrevlog(repo, 'perfrevlogchunks', file_, opts)
+    segmentforrevs = rl._chunkraw
 
     # Verify engines argument.
     if engines:
@@ -918,22 +919,22 @@
     def doread():
         rl.clearcaches()
         for rev in revs:
-            rl._chunkraw(rev, rev)
+            segmentforrevs(rev, rev)
 
     def doreadcachedfh():
         rl.clearcaches()
         fh = rlfh(rl)
         for rev in revs:
-            rl._chunkraw(rev, rev, df=fh)
+            segmentforrevs(rev, rev, df=fh)
 
     def doreadbatch():
         rl.clearcaches()
-        rl._chunkraw(revs[0], revs[-1])
+        segmentforrevs(revs[0], revs[-1])
 
     def doreadbatchcachedfh():
         rl.clearcaches()
         fh = rlfh(rl)
-        rl._chunkraw(revs[0], revs[-1], df=fh)
+        segmentforrevs(revs[0], revs[-1], df=fh)
 
     def dochunk():
         rl.clearcaches()
@@ -1002,6 +1003,7 @@
         raise error.CommandError('perfrevlogrevision', 'invalid arguments')
 
     r = cmdutil.openrevlog(repo, 'perfrevlogrevision', file_, opts)
+    segmentforrevs = r._chunkraw
     node = r.lookup(rev)
     rev = r.rev(node)
 
@@ -1033,7 +1035,7 @@
     def doread(chain):
         if not cache:
             r.clearcaches()
-        r._chunkraw(chain[0], chain[-1])
+        segmentforrevs(chain[0], chain[-1])
 
     def dorawchunks(data, chain):
         if not cache:
@@ -1061,7 +1063,7 @@
         r.revision(node)
 
     chain = r._deltachain(rev)[0]
-    data = r._chunkraw(chain[0], chain[-1])[1]
+    data = segmentforrevs(chain[0], chain[-1])[1]
     rawchunks = getrawchunks(data, chain)
     bins = r._chunks(chain)
     text = str(bins[0])