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