Improve IPv6 handling for host names

[pjbgf]

IPv6 host names are not being properly handled, which results in unhelpful error messages depending on the format used. Examples:

• git@[::ffff:8c52:7903]:go-git/go-git.git results in dial tcp: address [:22: missing ']' in address
• git@::ffff:8c52:7903:go-git/go-git.git results in dial tcp: lookup git@: no such host

This issue could be potentially tackled in two ways:

• (short-term) Adding error message highlighting that IPv6 is not supported for host names.
• (long-term) Add full support for IPv6 as host names.

[crazybolillo]

I was able to reproduce this with the following test (in remote_test.go):

func (s *RemoteSuite) TestFetchIPV6Endpoint(c *C) {
	r := NewRemote(nil, &config.RemoteConfig{Name: "foo", URLs: []string{"http://[2001:db8::]"}})
	err := r.Fetch(&FetchOptions{RemoteName: "foo"})
	c.Assert(err, IsNil)
}

The error is returned by res, err := s.client.Do(req.WithContext(ctx)) on plumbing/transport/http/common.go:55:

As far as my understanding goes the http.session object must be setup correctly to use tcp6 (in the case of https at least), still looking into it.