Merge branch 'jc/read-tree-empty-with-m' into maint

"git read-tree -m" (no tree-ish) gave a nonsense suggestion "use --empty if you want to clear the index". With "-m", such a request will still fail anyway, as you'd need to name at least one tree-ish to be merged. * jc/read-tree-empty-with-m: read-tree: "read-tree -m --empty" does not make sense