GIT_STRATEGY no-checkout

Description

I often find myself running git merge at the beginning of a build script. Also, I often find myself using just a few files from a repository, for example docker-compose.yml for a deploy from pre-built images. I feel it's a waste to check out all files of the repository in those cases.

Proposal

Add a GIT_STRATEGY no-checkout that skips the git checkout part.

It seems that git clone --no-checkout and git checkout are already separate operations. Adding this GIT_STRATEGY would then allow users to do all git operations themselves in the build script.