jj: Automatically show git diff for commit descriptions

vcs/jj/config/jj/config.toml

@@ -18,3 +18,16 @@ pager = "delta"
 
 [ui.diff]
 format = "git" # for the time being to use delta well
+
+[templates]
+draft_commit_description = '''
+concat(
+  description,
+  surround(
+    "\nJJ: This commit contains the following changes:\n", "",
+    indent("JJ:     ", diff.stat(72)),
+  ),
+  "\nJJ: ignore-rest\n",
+  diff.git(),
+)
+'''