- What I did
Removed git update-index from checking if a tree is dirty via Git.isDirty(). It was slow and not parallel safe. If anyone else - another linuxkit process or someone outside - runs anything that affects the index anywhere in the repo, it can be an issue.

Instead, use a series of other git commands to check if the tree is dirty.

Note that there is a slight chance that a file with no modifications except modtime could make it look dirty. That is an acceptable price to pay.

- How I did it

Changed git.go

- How to verify it
CI will be fine. I also did it manually, testing different scenarios of a package being dirty and clean in different ways.

- Description for the changelog

Ability to run multiple git calls, including pkg show-tag and pkg build, as well as other git commands, in parallel.