Drop rsync dependency

[jcbhmr]

We can use GIT_DIR and GIT_WORK_TREE to make the .git folder be somewhere completely different and the worktree root (the part where the files are to commit) be somewhere completely different too.

export GIT_DIR=/tmp/my.git
export GIT_WORK_TREE=my/subfolder

git init -b master
git add -A
git commit -m 'Add files!'

https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables

[jcbhmr]

Related to spenserblack/actions-wiki#72