Test if remote repository exists before cloning

When a repository does not exist on a remote, Gitaly won't be able to
clone it. This is correct behaviour, but from the clients perspective a
change in behaviour.

This change implements the client side changes that allows Gitaly to
execute a `git ls-remote <remote-url> HEAD`. This way the client has no
need to shell out to Git.

In the situation where multiple Gitalies are available, one is chosen at
random.

This commit closes https://gitlab.com/gitlab-org/gitlab-ce/issues/43929,
while its also a part of https://gitlab.com/gitlab-org/gitaly/issues/1084