setdiscovery: don't call "heads" wire command when heads specified

Our custom server has too many heads to announce (one per code review,
plus a public head), but it still lets the user request one of them by
doing

  hg pull -r <some expression>

After the client has resolved the expression to a set of nodeids by
calling the "lookup" wire command, it will start the discovery
phase. Before this patch, that doesn't take the requested heads into
account and unconditionally calls the server's "heads" command to find
all its heads. One consequence of that the "all remote heads known
locally" case triggers if the client already had the public head and
the user will see a "no changes found" message that's unrelated to the
head they requested. That message confused me for a while. More
imporantly, it also means that pullop.cgresult incorrectly (given our
arguably misbehaving server) gets set to 0 (no changesets added),
which confused some of our extensions.

This patch makes it so the client skips the "heads" command if the
user requested specific revisions.

Since the "heads" command is normally batched with the first "known"
command and calculating the list of heads is probably cheap, I don't
expect much improvement in speed from this.

Differential Revision: https://phab.mercurial-scm.org/D1962

mercurial/discovery.py

@@ -62,7 +62,7 def findcommonincoming(repo, remote, hea
         if allknown:
             return (heads, False, heads)
 
-    res = setdiscovery.findcommonheads(repo.ui, repo, remote,
+    res = setdiscovery.findcommonheads(repo.ui, repo, remote, heads,
                                        abortwhenunrelated=not force,
                                        ancestorsof=ancestorsof)
     common, anyinc, srvheads = res

mercurial/setdiscovery.py

@@ -130,7 +130,7 def _limitsample(sample, desiredlen):
         sample = set(random.sample(sample, desiredlen))
     return sample
 
-def findcommonheads(ui, local, remote,
+def findcommonheads(ui, local, remote, heads=None,
                     initialsamplesize=100,
                     fullsamplesize=200,
                     abortwhenunrelated=True,
@@ -155,6 +155,10 def findcommonheads(ui, local, remote,
     sample = _limitsample(ownheads, initialsamplesize)
     # indices between sample and externalized version must match
     sample = list(sample)
+    if heads:
+        srvheadhashes = heads
+        yesno = remote.known(dag.externalizeall(sample))
+    else:
     batch = remote.iterbatch()
     batch.heads()
     batch.known(dag.externalizeall(sample))