Merge branch 'rs/branch-del-symref'

A symbolic ref refs/heads/SYM was not correctly removed with
"git branch -d SYM"; the command removed the ref pointed by
SYM instead.

* rs/branch-del-symref:
  branch: show targets of deleted symrefs, not sha1s
  branch: skip commit checks when deleting symref branches
  branch: delete symref branch, not its target
  branch: factor out delete_branch_config()
  branch: factor out check_branch_commit()