Merge branch 'js/rebase-r-safer-label'

Merge branch 'js/rebase-r-safer-label'

A label used in the todo list that are generated by "git rebase
--rebase-merges" is used as a part of a refname; the logic to come
up with the label has been tightened to avoid names that cannot be
used as such.

* js/rebase-r-safer-label:
  rebase -r: let `label` generate safer labels
  rebase-merges: move labels' whitespace mangling into `label_oid()`