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

diff --cc Documentation/config.txt
Simple merge

diff --cc advice.c
Simple merge

diff --cc builtin/add.c
Simple merge