fix(smartlog) smartlog does not panic when run in a non-git directory

[cshinaver]

Closes #1069

First time contributing! tackling one of the smaller issues before I look into something more major.

[arxanas]

Hi @cshinaver, thanks for the PR! As written, this would only handle the panic for the case of git branchless smartlog, and only when failing to open the Repo. I would prefer to see an approach that can handle more panics. The best approach overall might be to replace EyreExitOr with an enum something like this:

enum CommandResult<T> {
    Ok(T),
    Exit { exit_code: isize, message: String },
    Unhandled(eyre::Error),
}

Then we could update the try_exit_code! macro (or create a new macro) that can wrap the result of Repo::from_dir and produce a CommandResult::Exit instead of letting the error propagate normally; and update do_main_and_drop_locals to handle the CommandResult and print the error message from the wrapped function.