--- a/mercurial/destutil.py	Thu Oct 15 01:35:44 2015 +0100
+++ b/mercurial/destutil.py	Thu Oct 15 01:11:00 2015 +0100
@@ -91,3 +91,57 @@
                     raise error.UpdateAbort(msg, hint=hint)
 
     return rev, movemark, activemark
+
+def destmerge(repo):
+    if repo._activebookmark:
+        bmheads = repo.bookmarkheads(repo._activebookmark)
+        curhead = repo[repo._activebookmark].node()
+        if len(bmheads) == 2:
+            if curhead == bmheads[0]:
+                node = bmheads[1]
+            else:
+                node = bmheads[0]
+        elif len(bmheads) > 2:
+            raise error.Abort(_("multiple matching bookmarks to merge - "
+                "please merge with an explicit rev or bookmark"),
+                hint=_("run 'hg heads' to see all heads"))
+        elif len(bmheads) <= 1:
+            raise error.Abort(_("no matching bookmark to merge - "
+                "please merge with an explicit rev or bookmark"),
+                hint=_("run 'hg heads' to see all heads"))
+    else:
+        branch = repo[None].branch()
+        bheads = repo.branchheads(branch)
+        nbhs = [bh for bh in bheads if not repo[bh].bookmarks()]
+
+        if len(nbhs) > 2:
+            raise error.Abort(_("branch '%s' has %d heads - "
+                               "please merge with an explicit rev")
+                             % (branch, len(bheads)),
+                             hint=_("run 'hg heads .' to see heads"))
+
+        parent = repo.dirstate.p1()
+        if len(nbhs) <= 1:
+            if len(bheads) > 1:
+                raise error.Abort(_("heads are bookmarked - "
+                                   "please merge with an explicit rev"),
+                                 hint=_("run 'hg heads' to see all heads"))
+            if len(repo.heads()) > 1:
+                raise error.Abort(_("branch '%s' has one head - "
+                                   "please merge with an explicit rev")
+                                 % branch,
+                                 hint=_("run 'hg heads' to see all heads"))
+            msg, hint = _('nothing to merge'), None
+            if parent != repo.lookup(branch):
+                hint = _("use 'hg update' instead")
+            raise error.Abort(msg, hint=hint)
+
+        if parent not in bheads:
+            raise error.Abort(_('working directory not at a head revision'),
+                             hint=_("use 'hg update' or merge with an "
+                                    "explicit revision"))
+        if parent == nbhs[0]:
+            node = nbhs[-1]
+        else:
+            node = nbhs[0]
+    return repo[node].rev()