diff -r edc8504bc26b -r 4cabeea6d214 hgext/hooklib/reject_merge_commits.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hgext/hooklib/reject_merge_commits.py	Sat Sep 07 14:50:39 2019 +0200
@@ -0,0 +1,45 @@
+# Copyright 2020 Joerg Sonnenberger <joerg@bec.de>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+"""reject_merge_commits is a hook to check new changesets for merge commits.
+Merge commits are allowed only between different branches, i.e. merging
+a feature branch into the main development branch. This can be used to
+enforce policies for linear commit histories.
+
+Usage:
+  [hooks]
+  pretxnchangegroup.reject_merge_commits = \
+    python:hgext.hooklib.reject_merge_commits.hook
+"""
+
+from __future__ import absolute_import
+
+from mercurial.i18n import _
+from mercurial import (
+    error,
+    pycompat,
+)
+
+
+def hook(ui, repo, hooktype, node=None, **kwargs):
+    if hooktype != b"pretxnchangegroup":
+        raise error.Abort(
+            _(b'Unsupported hook type %r') % pycompat.bytestr(hooktype)
+        )
+
+    ctx = repo.unfiltered()[node]
+    for rev in repo.changelog.revs(start=ctx.rev()):
+        rev = repo[rev]
+        parents = rev.parents()
+        if len(parents) < 2:
+            continue
+        if all(repo[p].branch() == rev.branch() for p in parents):
+            raise error.Abort(
+                _(
+                    b'%s rejected as merge on the same branch. '
+                    b'Please consider rebase.'
+                )
+                % rev
+            )