Take whole key in $key instead of $section and $key

`git::config` used to split the resource name (e.g. `user.email`) into `section` and `key`, only to pass them to `git_config`, where they were joined again. The regexes allowed only one dot, while `git config` allows more (e.g. `git-up.rebase.auto`).

This commit makes `git_config` take a single `key` parameter instead of `key` and `section`. `git::config` takes the whole key in `key` too, but can also take `section` for backward compatibility.