diff: replace --merge option by config option I can't think of any reason you'd want to enable the merge diff on a run-to-run basis; you'd probably either always or never want it set (though I can't see why you'd never want it set). If you have it set, you'll probably also want the same output in `hg log -p` output. Having a single config option for the feature makes sense. Differential Revision: https://phab.mercurial-scm.org/D9956

--- a/mercurial/configitems.py	Thu Dec 24 11:21:23 2020 -0500
+++ b/mercurial/configitems.py	Thu Feb 04 13:05:51 2021 -0800
@@ -754,6 +754,12 @@
 )
 _registerdiffopts(section=b'diff')
 coreconfigitem(
+    b'diff',
+    b'merge',
+    default=False,
+    experimental=True,
+)
+coreconfigitem(
     b'email',
     b'bcc',
     default=None,