Commit Briefs
add the required "headers" for a package
Emacs 27.1 is a prerequisite because of flatten-list, otherwise it should run on 25.1 onwards.
implement unregister
Even if it isn't really useful (it's only used in vc-transfer-file AFAICS) but it's easy to do and maybe can help someone eventually. See also github pr #7 for more background.
mark find-ignore-file as done and move it to the right place
vc.el doesn't document find-ignore-file, but it's used in the default `ignore' and `ignore-completion-table' functions.