discovery: abort also when pushing multiple headed new branch

Before this patch, pushing with --new-branch permits to create
multiple headed branch on the destination repository.

But permitting to create new branch should be different from
permitting to create multiple heads on branch.

This patch prevents from careless pushing multiple headed new branch,
and requires --force to push such branch forcibly.

mercurial/discovery.py

@@ -269,13 +269,12 b' def checkheads(repo, remote, outgoing, r'
     allfuturecommon = set(c.node() for c in repo.set('%ld', outgoing.common))
     allfuturecommon.update(allmissing)
     for branch, heads in sorted(headssum.iteritems()):
-        if heads[0] is None:
-            # Maybe we should abort if we push more that one head
-            # for new branches ?
-            continue
         candidate_newhs = set(heads[1])
         # add unsynced data
-        oldhs = set(heads[0])
+        if heads[0] is None:
+            oldhs = set()
+        else:
+            oldhs = set(heads[0])
         oldhs.update(heads[2])
         candidate_newhs.update(heads[2])
         dhs = None
@@ -310,7 +309,16 b' def checkheads(repo, remote, outgoing, r'
             newhs = candidate_newhs
         if [h for h in heads[2] if h not in discardedheads]:
             unsynced = True
-        if len(newhs) > len(oldhs):
+        if heads[0] is None:
+            if 1 < len(newhs):
+                dhs = list(newhs)
+                if error is None:
+                    error = (_("push creates multiple headed new branch '%s'")
+                             % (branch))
+                    hint = _("merge or"
+                             " see \"hg help push\" for detail about"
+                             " pushing new heads")
+        elif len(newhs) > len(oldhs):
             # strip updates to existing remote heads from the new heads list
             dhs = sorted(newhs - bookmarkedheads - oldhs)
         if dhs:

tests/test-push-warn.t

@@ -354,6 +354,29 b' Using --new-branch to push new named bra'
   adding file changes
   added 1 changesets with 1 changes to 1 files
 
+Pushing muliple headed new branch:
+
+  $ echo 14 > foo
+  $ hg -q branch f
+  $ hg -q ci -m 14
+  $ echo 15 > foo
+  $ hg -q ci -m 15
+  $ hg -q up 14
+  $ echo 16 > foo
+  $ hg -q ci -m 16
+  $ hg push --branch f --new-branch ../f
+  pushing to ../f
+  searching for changes
+  abort: push creates multiple headed new branch 'f'
+  (merge or see "hg help push" for detail about pushing new heads)
+  [255]
+  $ hg push --branch f --new-branch --force ../f
+  pushing to ../f
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 3 changesets with 3 changes to 1 files (+1 heads)
 
 Checking prepush logic does not allow silently pushing
 multiple new heads: