Keep the reserved info string beside the fence it spells, and one fenced-block spelling for both

src/backtick-runs.ts

@@ -1,3 +1,8 @@
+export function fencedCodeBlock(info: string, body: string): string {
+  const fence = '`'.repeat(Math.max(3, longestBacktickRun(body) + 1))
+  return body === '' ? `${fence}${info}\n${fence}` : `${fence}${info}\n${body}\n${fence}`
+}
+
 export function longestBacktickRun(text: string): number {
   let longest = 0
   let current = 0