[PATCH 2 of 2] repoview: bypass changelog method to computed cache key

[Pierre-Yves David]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at fb.com>
# Date 1449267735 28800
#      Fri Dec 04 14:22:15 2015 -0800
# Node ID 43caa4b5b82cb482a59513ee78b5bd96abefb7cd
# Parent  8daa6d98b6f67d39389bbd91d0a034466e1c965d
# EXP-Topic clcachekey
# Available At http://hg.netv6.net/marmoute-wip/mercurial/
#              hg pull http://hg.netv6.net/marmoute-wip/mercurial/ -r 43caa4b5b82c
repoview: bypass changelog method to computed cache key

Getting the data necessary for the cache key using the changelog/revlog method
adds a significant overhead. Given how simple the underlying implementation is
and often this code is ran, it make sense to violate layering and directly
compute the data.

Testing `hg log` on Mozilla-central, this reduce the time spent on changelog
cache validation by an extra half:

before: 12.2s of 69s
after:   6.1s of 62s

Total speed up from this patch and it's parent is 3x

(With stupid python profiler overhead)

The global speedup without profiler overhead is still there,

Before: 51s
After:  39s (-23%)

diff --git a/mercurial/repoview.py b/mercurial/repoview.py
--- a/mercurial/repoview.py
+++ b/mercurial/repoview.py
@@ -298,17 +298,20 @@ class repoview(object):
 
         this changelog must not be used for writing"""
         # some cache may be implemented later
         unfi = self._unfilteredrepo
         unfichangelog = unfi.changelog
+        # bypass call to changelog.method
+        unfiindex = unfichangelog.index
+        unfilen = len(unfiindex) - 1
+        unfinode = unfiindex[unfilen - 1][7]
+
         revs = filterrevs(unfi, self.filtername)
         cl = self._clcache
-        newkey = (len(unfichangelog), unfichangelog.tip(), hash(revs),
-                  unfichangelog._delayed)
-        if cl is not None:
-            if newkey != self._clcachekey:
-                cl = None
+        newkey = (unfilen, unfinode, hash(revs), unfichangelog._delayed)
+        if cl is not None and newkey != self._clcachekey:
+            cl = None
         # could have been made None by the previous if
         if cl is None:
             cl = copy.copy(unfichangelog)
             cl.filteredrevs = revs
             object.__setattr__(self, '_clcache', cl)