Implement `GitUrl` without user facing lifetimes

[tjtelan]

From #66

Longer term it might be wise to re-visit the use of a lifetime for the purpose of re-using slices of the input string - the url::Url crate does not bother with such micro-optimizations either, presumably for good reasons.

The url crate does their own kind of slicing without resulting in managing lifetimes on the public Url struct. https://github.com/servo/rust-url/blob/main/url/src/lib.rs#L228-L250

As a user, this feels nicer to use, but it will take some effort to implement