Investigate whether it's possible to use git fetch for GitLab.

Currently using `GIT_STRATEGY: fetch` (the default) results in cwtools-action failing on GitLab. This is because reviewdog needs both the source and target branches of the merge request to be present for the `git diff` it does.

Git clone seems to solve this, and is more reliable. However, for large mods it can be quite slow to clone even just the last 50 commits, so it would be preferable to use git fetch.

This issue tracks any progress towards that. Please emote if this is something that affects you!