Package sourcecache provides a cache of code found in Git repositories. It takes directly to the Gerrit instance at go.googlesource.com. If RegisterGitMirrorDial is called, it will first try to get code from gitmirror before falling back on Gerrit.
GetSourceTgz returns a Reader that provides a tgz of the requested source revision. repo is go.googlesource.com repo ("go", "net", etc) rev is git revision.
RegisterGitMirrorDial registers a dial function which will be used to reach gitmirror. If used, this function must be called before GetSourceTgz.