jj: update default description to include a diff

.config/jj/config.toml

···
 
 [template-aliases]
 'format_short_signature(signature)' = 'signature'
+
+[templates]
+draft_commit_description = '''
+concat(
+  description,
+  surround(
+    "\nJJ: This commit contains the following changes:\n", "",
+    indent("JJ:     ", diff.summary()),
+  ),
+  "\nJJ: ignore-rest\n",
+  indent("  " ,diff.git()),
+)
+'''