kart upgrade command is too eager to help

[olsen232]

Kart's logic for determining what type and version of repository it is operating needs improvement - if a repo mostly lacks internal Kart markers, Kart is too willing to decide that it is a Kart V1 repository and that it needs upgrading. One way to reproduce:

$ kart clone https://github.com/koordinates/kart.git 
$ cd kart
$ kart log
Error: This Kart repo uses Datasets v1 ...

(Note that koordinates/kart is a Git repo, not a Kart repo).

This is a particularly poor guess on Kart's part now that there are basically no V1 repos in existence outside of test data, so guessing the opposite would basically always be right.

This only happens when using Kart to clone the repo - if Git is used to do the cloning operation, Kart will correctly error with:
Error: Current directory is not an existing Kart repository

[olsen232]

Fixed by #920