feat: add ignore options to copy promotion step

[krancour]

@hiddeco and I discussed offline and found that recently replacing git-overwrite with git-clear, which we both felt strongly was an overall improvement, created one small difficulty.

git-overwrite knew to ignore a .git file (from a working tree of a bare repo) or .git directory when copying into a Git working tree. copy does not know to do that.

We discussed adding a configurable list of paths to ignore during the copy operation and including .git in that list by default.