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

diff --cc branch.c
Simple merge

diff --cc builtin/branch.c
Simple merge

diff --cc builtin/merge.c
Simple merge

diff --cc builtin/tag.c
Simple merge

diff --cc builtin/worktree.c
Simple merge

diff --cc gitweb/gitweb.perl
Simple merge

diff --cc refs.c
Simple merge

diff --cc refs.h
Simple merge

diff --cc t/t5604-clone-reference.sh
Simple merge