When running upgrade command, ignore errors upgrading sources if outside of a git repository

[noahd1]

If you run the qlty upgrade command outside of a git directory it currently will perform the upgrade but then raise the following error:

❌ This subcommand must be run within a Git repository.

Turns out this is because it is attempting to update sources, and updating sources requires being inside of a Git repository.

This PR prevents error propagation in this case, allowing source updating to occur within a Git repository, and otherwise allows the upgrade to complete successfully without an error message.

[brynary]

@noahd1 the source upgrade behavior is valuable, so I think we need to account for that.

When run outside of of an initialized project, we should just update the CLI. When run inside of an initialized project, we should perform the current behavior to upgrade the CLI and then upgrade the sources

[lsegal]

Would it be sufficient to just use the following? Not sure if this will generate garbage in logs or not.

let _ = SourceUpgrade::new().run();

[brynary]

@lsegal I think you may be right... @noahd1 it would be worth testing.

One small tweak, rather than the let _ = ... syntax, this would be more Rust idiomatic: SourceUpgrade::new().run().ok()

[noahd1]

Tried it. It works. 5 character addition FTW.

Updated the title and the description to more properly reflect the decided upon solution.