Fix tilde, backtick in fenced code info string

Related to mdx-js/mdx#630.
Closes GH-421.
Closes GH-422.

Reviewed-by: Marouane Fazouane <fazouanem3@gmail.com>
Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: John Otander <johnotander@gmail.com>

packages/remark-parse/lib/tokenize/code-fenced.js

@@ -100,8 +100,7 @@ function fencedCode(eat, value, silent) {
 
     if (
       character === lineFeed ||
-      character === tilde ||
-      character === graveAccent
+      (marker === graveAccent && character === marker)
     ) {
       break
     }

packages/remark-stringify/lib/visitors/code.js

@@ -8,6 +8,8 @@ module.exports = code
 
 var lineFeed = '\n'
 var space = ' '
+var tilde = '~'
+var graveAccent = '`'
 
 // Stringify code.
 // Creates indented code when:
@@ -73,6 +75,12 @@ function code(node, parent) {
     return pad(value, 1)
   }
 
+  // Backticks in the info string don’t work with backtick fenced code.
+  // Backticks (and tildes) are fine in tilde fenced code.
+  if (marker === graveAccent && info.indexOf(graveAccent) !== -1) {
+    marker = tilde
+  }
+
   fence = repeat(marker, Math.max(streak(value, marker) + 1, 3))
 
   return fence + info + lineFeed + value + lineFeed + fence

test/fixtures/input/fenced-code-info-with-marker.text

@@ -0,0 +1,28 @@
+If the info string comes after a backtick fence, it may not contain any backtick
+characters.
+The reason for this restriction is that otherwise some inline code would be
+incorrectly interpreted as the beginning of a fenced code block.
+
+Tildes in info strings are fine:
+
+```js filename=~/.bashrc
+console.log(1)
+```
+
+Tildes in info strings are fine after tilde fences:
+
+~~~js filename=~/.bashrc
+console.log(1)
+~~~
+
+Backticks in info strings are fine after tilde fences:
+
+~~~js title=`.bashrc`
+console.log(1)
+~~~
+
+Backticks in info strings are **not** fine after backtick fences:
+
+```js title=`.bashrc`
+console.log(1)
+```