changegroup: use any node, not min(), in treemanifest's generatemanifests

This is fixing quadratic behavior, which is probably not noticeable in the
common case, but if a very large directory gets added here, it can get pretty
bad. This was noticed because we had some pushes that spent >25s in changegroup
generation calling min() here, according to profiling.

The original reasoning for min() being used in 829d369fc5a8 was that, at that
point in the series, we were adding almost everything to tmfnodes during the
first iteration through the loop , so we needed to avoid sending child
directories before parents. Later changes made it so that the child directories
were added only when we visited the parent directory (not all of them on the
first iteration), so this is no longer necessary - there won't be any child
directories in tmfnodes before the parents have been sent.

This does mean that the manifests are now exchanged unordered, whereas
previously we would essentially do [a, b, b/c, b/c/d, e], we now can send a, b,
and e in any order; b/c must still follow b, and b/c/d must still follow b/c.

Differential Revision: https://phab.mercurial-scm.org/D1351

mercurial/changegroup.py

@@ -692,7 +692,7 b' class cg1packer(object):'
         # Callback for the manifest, used to collect linkrevs for filelog
         # revisions.
         # Returns the linkrev node (collected in lookupcl).
-        def makelookupmflinknode(dir):
+        def makelookupmflinknode(dir, nodes):
             if fastpathlinkrev:
                 assert not dir
                 return mfs.__getitem__
@@ -713,7 +713,7 b' class cg1packer(object):'
                 the client before you can trust the list of files and
                 treemanifests to send.
                 """
-                clnode = tmfnodes[dir][x]
+                clnode = nodes[x]
                 mdata = mfl.get(dir, x).readfast(shallow=True)
                 for p, n, fl in mdata.iterentries():
                     if fl == 't': # subdirectory manifest
@@ -733,15 +733,13 b' class cg1packer(object):'
 
         size = 0
         while tmfnodes:
-            dir = min(tmfnodes)
-            nodes = tmfnodes[dir]
+            dir, nodes = tmfnodes.popitem()
             prunednodes = self.prune(dirlog(dir), nodes, commonrevs)
             if not dir or prunednodes:
                 for x in self._packmanifests(dir, prunednodes,
-                                             makelookupmflinknode(dir)):
+                                             makelookupmflinknode(dir, nodes)):
                     size += len(x)
                     yield x
-            del tmfnodes[dir]
         self._verbosenote(_('%8.i (manifests)\n') % size)
         yield self._manifestsdone()