Markdown writer: Ensure the code fence is long enough.

Previously too few backticks were used when the code block
contained an indented line of backticks.  (Ditto tildes.)

Cloess #5519.

test/command/5519.md

@@ -0,0 +1,14 @@
+~~~
+% pandoc -t markdown
+``````{.attr}
+ ```
+ code
+ ```
+``````
+^D
+```` {.attr}
+ ```
+ code
+ ```
+````
+~~~