`git_repo` should have a fallback/workaround for 'dumb' git servers

As an example:

   [--:--:--] START   ctrl.bst:freedesktop-sdk.bst:components/libmnl.bst: Fetching from https://git.netfilter.org/libmnl
    [--:--:--] STATUS  ctrl.bst:freedesktop-sdk.bst:components/libmnl.bst: Fetching 493aacf2ec9cc61a5b30d77cd55ec248f033bc74
    [--:--:--] WARNING ctrl.bst:freedesktop-sdk.bst:components/libmnl.bst: not implemented <bound method AbstractHttpGitClient.fetch_pack of Urllib3HttpGitClient('https://git.netfilter.org/libmnl/', dumb=True)>
    [00:00:00] FAILURE ctrl.bst:freedesktop-sdk.bst:components/libmnl.bst: Fetching from https://git.netfilter.org/libmnl
    [00:00:00] FAILURE [7fa087ba] ctrl.bst:freedesktop-sdk.bst:components/libmnl.bst: Fetching from a dumb repository is not currently supported, please set up a mirror on a smart server

It's good that a suggestion is permeated with the failure, however it would be good to support these servers (in this case https://git.netfilter.org/) without a downstream user having to resort to a mirror, or a different git plugin. In this case yes fdsdk does provide a public mirror, but this won't always be the case for every user or source.