Idempotent init command

These changes make it possible to run git-bpf-init on a repo any number of times and the state of the repo will be as if it was run for the first time using the latest git-bpf code. It does this by removing any existing symlinks to targets within the git-bpf source location, removing aliases to git-bpf commands from the local git config, and removing the gitbpf section of the local git config.
The one assumption is that there will never be any reason to alter the way the rr-cache directory has been set up - this is just left alone if the repo was previously initialized for git-bpf.