skip unshallow if the repo is already unshallowed

[shogo82148]

I want to run some scripts before tagging.
e.g. https://github.com/reviewdog/action-actionlint/blob/d3a6e1d60834d9308636d291e53651ba1c2479d7/.github/workflows/release.yml#L30-L37

But I can't make any change in my script because of a permission error.

https://github.com/reviewdog/action-actionlint/runs/3135103508?check_suite_focus=true#step:5:193

+ git checkout main -- action.yml
+ perl -i -pe 's(image:\s*["'\'']?Dockerfile["'\'']?)(image: '\''docker://ghcr.io/reviewdog/action-actionlint:v1.5.0'\'')' action.yml
+ git add action.yml
+ git commit -m 'bump v1.5.0'
error: insufficient permission for adding an object to repository database .git/objects
error: insufficient permission for adding an object to repository database .git/objects
error: Error building trees
Error: Process completed with exit code 1.

It looks that files created in the docker environment doesn't have enough permission.
This patch stop creating new files by skipping git fetch if the repository is already unshallowed.

[haya14busa]

Thanks!

[github-actions]

🚀 [bumpr] Bumped! New version:v1.7.0 Changes:v1.6.1...v1.7.0