introduce generic git url alias (git_https)

Every source URL needs to use an alias, to enable mirroring. If there isn't a suitable one already, then people will tend to add a new one by adding it to aliases.yml, but they can easily forget to also put it in mirrors.yml.

Rather than have a bunch of single-use aliases added (and to avoid people forgetting to update mirrors.yml), it's better if we have a generic alias.

I've added "git_http:" and "git_https", which resolve to "http://" and "https://" respectively. These can be used for pretty much any url, without needing to create a lot of single-use aliases in future.

(Obviously, we can still add other new aliases as needed, if we think they'll be use for more than one url or if we have a some other need to add them.)