When "utf8mb4" is available, use it as the default client charset when invoking standalone "mysql" commands

Summary:
Fixes T13390. We have some old code which doesn't dynamically select between "utf8mb4" and "utf8". This can lead to dumping utf8mb4 data over a utf8 connection in `bin/storage dump`, which possibly corrupts some emoji/whales.

Instead, prefer "utf8mb4" if it's available.

Test Plan: Ran `bin/storage dump` and `bin/storage shell`, saw sub-commands select utf8mb4 as the client charset.

Maniphest Tasks: T13390

Differential Revision: https://secure.phabricator.com/D20742

src/infrastructure/storage/management/PhabricatorStorageManagementAPI.php

···
     return self::isCharacterSetAvailableOnConnection($character_set, $conn);
   }
 
+  public function getClientCharset() {
+    if ($this->isCharacterSetAvailable('utf8mb4')) {
+      return 'utf8mb4';
+    } else {
+      return 'utf8';
+    }
+  }
+
   public static function isCharacterSetAvailableOnConnection(
     $character_set,
     AphrontDatabaseConnection $conn) {

src/infrastructure/storage/management/workflow/PhabricatorStorageManagementDumpWorkflow.php

···
     $argv = array();
     $argv[] = '--hex-blob';
     $argv[] = '--single-transaction';
-    $argv[] = '--default-character-set=utf8';
+
+    $argv[] = '--default-character-set';
+    $argv[] = $api->getClientCharset();
 
     if ($args->getArg('for-replica')) {
       $argv[] = '--master-data';

src/infrastructure/storage/management/workflow/PhabricatorStorageManagementShellWorkflow.php

···
     }
 
     return phutil_passthru(
-      'mysql --protocol=TCP --default-character-set=utf8mb4 '.
-      '-u %s %C -h %s %C',
+      'mysql --protocol=TCP --default-character-set %R -u %s %C -h %s %C',
+      $api->getClientCharset(),
       $api->getUser(),
       $flag_password,
       $host,