Support dependencies as git url with exact commit

Git repositories can be directly used by dub as dependencies.

dub.json:
-------
{
    "name": "git-dependency",
    "dependencies": {
        "gitcompatibledubpackage": {
            "repository": "git+https://github.com/dlang-community/gitcompatibledubpackage.git",
            "version": "ccb31bf6a655437176ec02e04c2305a8c7c90d67"
        }
    }
}
-------