manifest: increase lrucache from 3 to 4

During a commit amend there are 4 manifests being handled:

- original commit
- temporary commit
- amended commit
- merge base

This causes a manifest cache miss which hurts perf on large repos. On a large
repo, this fix causes amend to go from 6 seconds to 5.5 seconds.

mercurial/manifest.py

@@ -30,8 +30,9 b' class manifestdict(dict):'
 
 class manifest(revlog.revlog):
     def __init__(self, opener):
-        # we expect to deal with not more than three revs at a time in merge
-        self._mancache = util.lrucachedict(3)
+        # we expect to deal with not more than four revs at a time,
+        # during a commit --amend
+        self._mancache = util.lrucachedict(4)
         revlog.revlog.__init__(self, opener, "00manifest.i")
 
     def parse(self, lines):