diff: add help text to highlight the ability to do "merge diffs"

Differential Revision: https://phab.mercurial-scm.org/D12605

mercurial/commands.py

@@ -2536,7 +2536,8 b' def diff(ui, repo, *pats, **opts):'
 
        :hg:`diff` may generate unexpected results for merges, as it will
        default to comparing against the working directory's first
-       parent changeset if no revisions are specified.
+       parent changeset if no revisions are specified.  To diff against the
+       conflict regions, you can use `--config diff.merge=yes`.
 
     By default, the working directory files are compared to its first parent. To
     see the differences from another revision, use --from. To see the difference

tests/test-help.t

@@ -644,7 +644,8 b' Test command without options'
       Note:
          'hg diff' may generate unexpected results for merges, as it will
          default to comparing against the working directory's first parent
-         changeset if no revisions are specified.
+         changeset if no revisions are specified.  To diff against the conflict
+         regions, you can use '--config diff.merge=yes'.
   
       By default, the working directory files are compared to its first parent.
       To see the differences from another revision, use --from. To see the