discovery: only calculate closed branches if required

The number of new closed branches is required for printing in error message. So
let's only calculate them if we need to print error about new branches.

Differential Revision: https://phab.mercurial-scm.org/D6314

mercurial/discovery.py

@@ -340,15 +340,15 b' def checkheads(pushop):'
     pushop.pushbranchmap = headssum
     newbranches = [branch for branch, heads in headssum.iteritems()
                    if heads[0] is None]
-    # Makes a set of closed branches
+    # 1. Check for new branches on the remote.
+    if newbranches and not newbranch:  # new branch requires --new-branch
+        branchnames = ', '.join(sorted(newbranches))
+        # Calculate how many of the new branches are closed branches
     closedbranches = set()
     for tag, heads, tip, isclosed in repo.branchmap().iterbranches():
         if isclosed:
             closedbranches.add(tag)
     closedbranches = (closedbranches & set(newbranches))
-    # 1. Check for new branches on the remote.
-    if newbranches and not newbranch:  # new branch requires --new-branch
-        branchnames = ', '.join(sorted(newbranches))
         if closedbranches:
             errmsg = (_("push creates new remote branches: %s (%d closed)!")
                         % (branchnames, len(closedbranches)))