fetch remote ref during update on git repository

[kirecek]

It seems getter.Get during update does not use the same ref as in clone command.

The repetitive call of .Get on url without specified ref

        src := "git::https://github.com/hashicorp/go-getter"
        dest := "/tmp/test"

	err := getter.Get(dest, src)
	if err != nil {
		log.Fatalf("get repo: %v", err)
	}

fails with the following error:

/usr/bin/git exited with 128: fatal: empty string is not a valid pathspec. please use . instead if you meant to match all paths

Is this by design or should the library match clone behaviour?