Fix URL check in build from Git.

Existing check ignores valid git URLs with prefix, "git@", "http://" or "https://". This leads to erroneously prepending "https://" to valid URLs.
`IsGIT` accepts URLs like `github.com/docker/docker`, which requires to be prefixed with a protocol before passed to git clone. Hence the introduction of this check. Anyway, I changed the function name to `ValidGITTransport` to better reflect its intent.