feat(cli-repl): only store non-default config keys on disk MONGOSH-794

[addaleax]

For context, the original motivation here was that for snippets,
if we change the default index URL, users should receive that change
if they have not explicitly changed the config themselves.

[rose-m]

👍 I somehow find the wording not entirely correct as it's more a "only store explicitly set config options to disk". For the "non-default" I'd had expected it to only store it to disk when the actual value diverges from the default setting.

[addaleax]

For the "non-default" I'd had expected it to only store it to disk when the actual value diverges from the default setting.

Yeah, I’m also not particularly sure what the expected behavior there is – if you set it to (a value that happens to be) the default value, and then we change the default, what would the right value afterwards?

[rose-m]

That's exactly what I was thinking about... but yeah I'm totally fine with the semantics of "if you set the value explicitely, it will stay like that".