bookmarks: prefetch 'lookup' outside of the loop

Skipping the attribute lookup up raise a significant speedup.

Example on a repository with about 4000 bookmarks.

Before:
! wall 0.026027 comb 0.020000 user 0.020000 sys 0.000000 (best of 112)
After:
! wall 0.021580 comb 0.020000 user 0.020000 sys 0.000000 (best of 134)

(This is also in its own changeset to clarify the perf win from another coming
changesets)

mercurial/bookmarks.py

@@ -50,6 +50,7 b' class bmstore(dict):'
     def __init__(self, repo):
         dict.__init__(self)
         self._repo = repo
+        lookup = repo.changelog.lookup
         try:
             bkfile = _getbkfile(repo)
             for line in bkfile:
@@ -63,7 +64,7 b' class bmstore(dict):'
                 sha, refspec = line.split(' ', 1)
                 refspec = encoding.tolocal(refspec)
                 try:
-                    self[refspec] = repo.changelog.lookup(sha)
+                    self[refspec] = lookup(sha)
                 except LookupError:
                     pass
         except IOError as inst: