Merge branch 'jc/maint-fetch-tighten-refname-check'

Merge branch 'jc/maint-fetch-tighten-refname-check'

For a fetch refspec (or the result of applying wildcard on one), we
always want the RHS to map to something inside "refs/" hierarchy.

This was split out from discarded jc/maint-push-refs-all topic.

* jc/maint-fetch-tighten-refname-check:
  get_fetch_map(): tighten checks on dest refs