New feature

In the manifest it would be nice if we could default to a git tag or commit hash.

Usage scenario

manifest {
    version         = '1.0.0'
    defaultBranch   = '1.0.0'
    // defaultRevision = '1.0.0'
}

So we don't need a master branch that is just a "stable" release, instead we can have users checkout a version by default and they won't get a dev version by mistake.

Suggest implementation

1. Add a defaultRevision or modify the defaultBranch functionality to accept tags or commit hashs. The same functionality as -r
2. Set the default behavior to look for a git tag, and if there isn't one default to master.
3. If a manifest.version is set, then look for that git tag instead of using the defaultBranch on the git repo.