follow-up: 59e9337be0
For some reason, without this change, the first build works but subsequent builds fail.
Not sure what the cause is. This is why Travis builds work.
If there is a collision between a branch name and a commit hash, git
will choose the branch, even if the full 40-hex-long commit hash is
given. GitHub disallows branches/tags with such a name but git itself
does not. By adding the `^{commit}` syntax sugar after a ref name,
we can tell git that we want the commit hash to be preferred,
and hence we don't need to trust GitHub (only git).
see https://security.stackexchange.com/questions/225411/
