Merge branch 'jc/forbid-head-as-tagname'

"git tag" has been taught to refuse to create refs/tags/HEAD as such a tag will be confusing in the context of UI provided by the Git Porcelain commands. * jc/forbid-head-as-tagname: tag: "git tag" refuses to use HEAD as a tagname t5604: do not expect that HEAD can be a valid tagname refs: drop strbuf_ prefix from helpers refs: move ref name helpers around