Add reference by commit hash support

[braydonf]

As there isn't currently a way to clone by arbitrary hash, only tags and branch heads are available when using git clone --depth 1 --branch <tag|branch> <repo>, it will be necessary to clone the entire repository and then navigate to the hash. This is inefficient, so such usage should be discouraged. However there may be some cases when it will be desirable, for example in implementing a lockfile. With the entire repository already mirrored locally, in a cache, it is less of an inefficiency.