cache `git config` output for repository to skip process calls

We have a number of places in the app where we read the config for a repository:

- check `protocol.version` before performing a `fetch/pull/push/clone`

https://github.com/desktop/desktop/blob/bee4c6bb74d8060c6514fad3ab6a3f366d4c8eaf/app/src/lib/git/core.ts#L325-L331

- check `trailer.separators` when extracting co-authors from a commit message

https://github.com/desktop/desktop/blob/bee4c6bb74d8060c6514fad3ab6a3f366d4c8eaf/app/src/lib/git/interpret-trailers.ts#L75-L79

- check `pull.rebase` to indicate what "pull" will do for the user

https://github.com/desktop/desktop/blob/bee4c6bb74d8060c6514fad3ab6a3f366d4c8eaf/app/src/lib/stores/git-store.ts#L339

These are run fairly often during typical usage of the app, and a way to optimize this could be:

- run a single `git config` and extract all key-value entries emitted by Git
- cache these values inside `GitStore` in some data structure
- refresh this cache on the same frequency as other repository data
- where an action requires configuration, pass in this state rather than launching an additional `git` process