manifest: make lru size configurable

On machines with lots of ram, it's beneficial to increase the lru size of the
manifest cache.  On a large repo, configuring the lru to be size 10 can shave a
large rebase (~12 commits) down from 95s to 70s.

mercurial/localrepo.py

@@ -323,6 +323,9 b' class localrepository(object):'
         maxchainlen = self.ui.configint('format', 'maxchainlen')
         if maxchainlen is not None:
             self.svfs.options['maxchainlen'] = maxchainlen
+        manifestcachesize = self.ui.configint('format', 'manifestcachesize')
+        if manifestcachesize is not None:
+            self.svfs.options['manifestcachesize'] = manifestcachesize
 
     def _writerequirements(self):
         reqfile = self.vfs("requires", "w")

mercurial/manifest.py

@@ -220,9 +220,14 b' def _parse(lines):'
 
 class manifest(revlog.revlog):
     def __init__(self, opener):
-        # we expect to deal with not more than four revs at a time,
-        # during a commit --amend
-        self._mancache = util.lrucachedict(4)
+        # During normal operations, we expect to deal with not more than four
+        # revs at a time (such as during commit --amend). When rebasing large
+        # stacks of commits, the number can go up, hence the config knob below.
+        cachesize = 4
+        opts = getattr(opener, 'options', None)
+        if opts is not None:
+            cachesize = opts.get('manifestcachesize', cachesize)
+        self._mancache = util.lrucachedict(cachesize)
         revlog.revlog.__init__(self, opener, "00manifest.i")
 
     def readdelta(self, node):