Merge branch 'jk/warn-add-gitlink'

Using "git add d/i/r" when d/i/r is the top of the working tree of a separate repository would create a gitlink in the index, which would appear as a not-quite-initialized submodule to others. We learned to give warnings when this happens. * jk/warn-add-gitlink: t: move "git add submodule" into test blocks add: warn when adding an embedded repository