bundlerepo: let bundle repo look in the _mancache

When looking up a base revision, we were ignoring the contents that were already
available in the manifest's _mancache. This patch allows us to use that data
instead of reading from the revlog.

This is useful in our pushrebase extension (which allows rebasing on the server
side during a push) because it allows us to prefetch the bundle base manifest
before aquiring the repo lock (1 second saving), which means doing less work inside the lock,
which means a 20% higher commit rate.

mercurial/bundlerepo.py

@@ -195,7 +195,15 b' class bundlemanifest(bundlerevlog, manif'
                               linkmapper)
 
     def baserevision(self, nodeorrev):
-        return manifest.manifest.revision(self, nodeorrev)
+        node = nodeorrev
+        if isinstance(node, int):
+            node = self.node(node)
+
+        if node in self._mancache:
+            result = self._mancache[node][0].text()
+        else:
+            result = manifest.manifest.revision(self, nodeorrev)
+        return result
 
 class bundlefilelog(bundlerevlog, filelog.filelog):
     def __init__(self, opener, path, bundle, linkmapper):