The change in commit  v3.8.0-rc2~7^2 (ExternalProject: Run `git
checkout` with `--` to clarify arguments, 2017-02-24) broke the case of
`GIT_TAG some-remote-branch` because an explicit `--` means that the
preceding argument is a tree-ish instead of a branch.  Revert the change
and leave a comment.

Fixes: #16763