Fix caching for synthetic inline comments

Test Plan: Looked at diff with several different lint errors, saw correct messages in their inline comments.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3827

src/applications/differential/storage/DifferentialInlineComment.php

@@ -126,7 +126,8 @@ final class DifferentialInlineComment
 
 
   public function getMarkupFieldKey($field) {
-    return 'DI:'.$this->getID();
+    // We can't use ID because synthetic comments don't have it.
+    return 'DI:'.PhabricatorHash::digest($this->getContent());
   }
 
   public function newMarkupEngine($field) {

src/infrastructure/markup/PhabricatorMarkupInterface.php

@@ -47,7 +47,7 @@ interface PhabricatorMarkupInterface {
    *   "{$phid}:{$field}"
    *
    * @param string Field name.
-   * @return string Cache key.
+   * @return string Cache key up to 125 characters.
    *
    * @task markup
    */