push: restrict common discovery to the pushed set

This changeset make use of the ability of the set discovery to only search
common changeset for a subset of the repository. Restricting that search to the
pushed set avoid potential waste of time finding out the status of many
unrelated related revision.

Repository with many heads were especially badly affected by this. Here is an
example of findcommonhead discovery for pushing 11 outgoing changeset on a
repository with tens of thousand of unrelated heads. (discovery run over a ssh
link to localhost).

Before:
    queries: 92
    time:    44.1996s

After:
    queries: 3
    time:    0.6938s

A x63 speedup even with a network link without latency.

mercurial/discovery.py

@@ -27,7 +27,7 b' from . import ('
     util,
 )
 
-def findcommonincoming(repo, remote, heads=None, force=False):
+def findcommonincoming(repo, remote, heads=None, force=False, ancestorsof=None):
     """Return a tuple (common, anyincoming, heads) used to identify the common
     subset of nodes between repo and remote.
 
@@ -38,6 +38,9 b' def findcommonincoming(repo, remote, hea'
       changegroupsubset. No code except for pull should be relying on this fact
       any longer.
     "heads" is either the supplied heads, or else the remote's heads.
+    "ancestorsof" if not None, restrict the discovery to a subset defined by
+      these nodes. Changeset outside of this set won't be considered (and
+      won't appears in "common")
 
     If you pass heads and they are all known locally, the response lists just
     these heads in "common" and in "heads".
@@ -60,7 +63,8 b' def findcommonincoming(repo, remote, hea'
             return (heads, False, heads)
 
     res = setdiscovery.findcommonheads(repo.ui, repo, remote,
-                                       abortwhenunrelated=not force)
+                                       abortwhenunrelated=not force,
+                                       ancestorsof=ancestorsof)
     common, anyinc, srvheads = res
     return (list(common), anyinc, heads or list(srvheads))
 
@@ -142,7 +146,8 b' def findcommonoutgoing(repo, other, only'
 
     # get common set if not provided
     if commoninc is None:
-        commoninc = findcommonincoming(repo, other, force=force)
+        commoninc = findcommonincoming(repo, other, force=force,
+                                       ancestorsof=onlyheads)
     og.commonheads, _any, _hds = commoninc
 
     # compute outgoing

mercurial/exchange.py

@@ -514,7 +514,11 b' def _pushdiscovery(pushop):'
 def _pushdiscoverychangeset(pushop):
     """discover the changeset that need to be pushed"""
     fci = discovery.findcommonincoming
-    commoninc = fci(pushop.repo, pushop.remote, force=pushop.force)
+    if pushop.revs:
+        commoninc = fci(pushop.repo, pushop.remote, force=pushop.force,
+                        ancestorsof=pushop.revs)
+    else:
+        commoninc = fci(pushop.repo, pushop.remote, force=pushop.force)
     common, inc, remoteheads = commoninc
     fco = discovery.findcommonoutgoing
     outgoing = fco(pushop.repo, pushop.remote, onlyheads=pushop.revs,