smarthttp: Verify repo existence before dealing with cache

InfoRefsUploadPack and InfoRefsReceivePack doesn't verify if repository exist before the caching get involved into the process. It requires additional operations to run that does nothing at the end because of missed repository. It also produces error log entries that may confuse some time. In order to optimise this case we first get path to the repository and if it doesn't exist the error is returned immediately without need to bother caching (and no extra logs).

The example of the logs without applied optimisation: