kiss: git caching

[dylanaraps]

1. If a repository does not exist in the sources cache it is initialized and then pulled down as shallowly as possible.
2. If the source tracks latest commit or a specific branch/tag, the package manager will pull changes every build.
3. If the source tracks a specific commit, the commit will be pulled once and every subsequent build will only do a checkout.

Bugs:

• Does not handle cases where remote changes.
• Git runs in checksum mode.