clonebundle: add a `filter_bundle_url` function This function does nothing by default, but give extension the opportunity to alter the URL, typically, this could be used to inject authentication token when serving clone bundle for private repositories.

clonebundles: move the manifest reading in a dedicated function We are about to make the logic more advanced to help hosting solution, so we need to centralize it first.

doc: format argument for date uses strftime format string (issue6818)