Allow user to specify a different refspec for remote tracking branches

[draftcode]

In some cases, users do not want to keep the remote tracking branches small, e.g. sync only the trunk branch. This allows the user to avoid accidentally checking out a branch from a remote tracking branch.

av-cli has been depending on the remote tracking branch. Initially, it's been assuming that it's +refs/heads/*:refs/remotes/origin/*, but now we take the remote name as well as using the refspec properly to do a map/reverse-map the remote tracking branches.

Going further, we can make av-cli to take a different refspec for its own usage in the config. This would make most of the tools ignore those refs.