Support main branch in bot_update

Change-Id: I58e88e9505779c309ac0af9d5b78b45feaa75543
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2528669Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

recipes/recipe_modules/bot_update/api.py

@@ -444,7 +444,7 @@ class BotUpdateApi(recipe_api.RecipeApi):
       return 'HEAD'
 
     target_ref = self.m.tryserver.gerrit_change_target_ref
-    if target_ref == 'refs/heads/master':
+    if target_ref in ['refs/heads/master', 'refs/heads/master']:
       return 'HEAD'
 
     return target_ref