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

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