Merge branch 'ph/use-delete-refs'

When removing many branches and tags, the code used to do so one
ref at a time.  There is another API it can use to delete multiple
refs, and it makes quite a lot of performance difference when the
refs are packed.

* ph/use-delete-refs:
  use delete_refs when deleting tags or branches

diff --cc builtin/branch.c
Simple merge

diff --cc builtin/tag.c
Simple merge