brew-test-bot: force checkout master with cleanup.

Library/Contributions/cmd/brew-test-bot.rb

@@ -219,6 +219,7 @@ class Test
     git 'stash'
     git 'am --abort 2>/dev/null'
     git 'rebase --abort 2>/dev/null'
+    git 'checkout -f master'
     git 'reset --hard'
     git 'clean --force -dx'
   end