Merge branch 'mv/maint-branch-m-symref'

Merge branch 'mv/maint-branch-m-symref'

* mv/maint-branch-m-symref:
  update-ref --no-deref -d: handle the case when the pointed ref is packed
  git branch -m: forbid renaming of a symref
  Fix git update-ref --no-deref -d.
  rename_ref(): handle the case when the reflog of a ref does not exist
  Fix git branch -m for symrefs.