Autoload cache must be cleared before versioncontrol_git_update_6204() can run

Otherwise the classes introduced in #1291308: Add setDefaultBranch() and getDefaultBranch() method to VersioncontrolGitRepository won't be available and the update function will fail. I didn't catch that because I probably cleared caches during development, but the function failed when I tested it on a fresh install (that is my d.o staging site).