cache remotes for repository in GitStore to avoid lookups

As part of investigating opportunities for performance improvement, I pointed out that we call `getRemotes` in several places in the app and do not cache these values.

https://github.com/desktop/desktop/blob/bee4c6bb74d8060c6514fad3ab6a3f366d4c8eaf/app/src/lib/stores/app-store.ts#L841

https://github.com/desktop/desktop/blob/bee4c6bb74d8060c6514fad3ab6a3f366d4c8eaf/app/src/lib/stores/app-store.ts#L4921

https://github.com/desktop/desktop/blob/bee4c6bb74d8060c6514fad3ab6a3f366d4c8eaf/app/src/lib/stores/git-store.ts#L905

https://github.com/desktop/desktop/blob/bee4c6bb74d8060c6514fad3ab6a3f366d4c8eaf/app/src/lib/stores/git-store.ts#L1067

https://github.com/desktop/desktop/blob/bee4c6bb74d8060c6514fad3ab6a3f366d4c8eaf/app/src/lib/stores/git-store.ts#L1104

https://github.com/desktop/desktop/blob/bee4c6bb74d8060c6514fad3ab6a3f366d4c8eaf/app/src/lib/stores/git-store.ts#L1435

Remotes change infrequently, and Desktop does very little work itself to add or remove them, so I think we can cache these values to reduce the work of spawning processes when we need to inspect the remotes for a repository. This will be more beneficial for our Windows users, because of the overhead of spawning processes is more expensive there.