git-fat should support a way to guess fat store URLs

[OptiverTimAll]

I've been investigating git large-file storage solutions, and while git-fat is more generally useful, GitHub's git-lfs is a bit nicer to use. One reason is that you don't always have to explicitly tell it where your external file-storage is; if you're using GitHub Enterprise, the file-storage URL is at a predictable location relative to the upstream repo URL. This is not the case for git-fat in general, but it is probably true for most people using git-fat within an organisation.

I want to be able to put rewrite rules into ~/.gitconfig that git-fat can use to find the fat store. For example, if I run git clone ssh://git@git/repos/myproject.git on my workstation, I may know that the fat store is always going to be rsync://git@storage/git-fat/myproject and set up a rewrite rule accordingly.

Following the example of remote.<name>.fetch, such a rewrite rule might look like:

[git-fat-rewrite "common projects"]
    clone_url = ssh://git@git/repos/*.git
    storage_url = rsync://git@storage/git-fat/*

Alternatively, following the example of url.<base>.insteadOf, such a rewrite rule might look like:

[git-fat-storage "ssh://git@git/repos"]
    pull = rsync://git@storage/git-fat

Those aren't the only two options, of course; just starting-points for discussion.