push should not allow to push diverged bookmark

[Marc Strapetz]

When having a bookmark for which there exists a new remote commit, it's 
still possible to push a diverged local commit without --force option. I 
think it's more reasonable to disallow such pushes, similar as when 
pushing a branch or a specific revision. The patch below should resolve 
the problem.

-Marc

# HG changeset patch
# User Marc Strapetz <marc.strapetz at syntevo.com>
# Date 1438690573 -7200
#      Tue Aug 04 14:16:13 2015 +0200
# Node ID 02624e707a527ded09612539b8e7fc26889679f7
# Parent  eabba9c75061254ff62827f92df0f32491c74b3d
push: disallow pushing diverged bookmark using "hg push <bookmark>" 
syntax without --force

diff -r eabba9c75061 -r 02624e707a52 mercurial/discovery.py
--- a/mercurial/discovery.py	Fri Jun 26 18:45:29 2015 -0500
+++ b/mercurial/discovery.py	Tue Aug 04 14:16:13 2015 +0200
@@ -265,7 +265,7 @@
              if bookmarks.validdest(repo, rctx, lctx):
                  bookmarkedheads.add(lctx.node())
          else:
-            if bm in newbookmarks:
+            if bm in newbookmarks and not bm in remotebookmarks:
                  bookmarkedheads.add(repo[bm].node())

      # 3. Check for new heads.