Git is a transport and storage mechanism for us so we don't want to
allow arbitrary customisation of this due to user issues e.g. #3664
and #3561.
Fixes#3664.
Add new `brew update-reset` command to provide a helpful troubleshooting
fallback to fetch and reset all repositories. This could have lived in
`brew update` but it makes sense to avoid the complexity of sharing
logic between these scripts and keeping this one simpler.
