Truncate the one-line diff update summary when updating a revision to 250 bytes

Summary:
Fixes T7899. If you create or update a revision and type an enormously long first line, we currently fatal trying to insert it into the database.

This text is only used to show a single-line summary of the diff in the "History" tab, which should probably be updated anyway. For now, stop fataling.

Test Plan:
Uploaded a diff with the description "MMMM..." (thousands of them).

Before patch: fatal on description being too long.
After patch: beautiful "MMMM" summary.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7899

Differential Revision: https://secure.phabricator.com/D17038

src/applications/differential/conduit/DifferentialConduitAPIMethod.php

@@ -124,8 +124,15 @@ abstract class DifferentialConduitAPIMethod extends ConduitAPIMethod {
     $message = $request->getValue('message');
     if (strlen($message)) {
       // This is a little awkward, and should maybe move inside the transaction
-      // editor. It largely exists for legacy reasons.
+      // editor. It largely exists for legacy reasons. See some discussion in
+      // T7899.
       $first_line = head(phutil_split_lines($message, false));
+
+      $first_line = id(new PhutilUTF8StringTruncator())
+        ->setMaximumBytes(250)
+        ->setMaximumGlyphs(80)
+        ->truncateString($first_line);
+
       $diff->setDescription($first_line);
       $diff->save();