Merge branch 'km/empty-repo-is-still-a-repo'

Running "git add" on a repository created inside the current
repository is an explicit indication that the user wants to add it
as a submodule, but when the HEAD of the inner repository is on an
unborn branch, it cannot be added as a submodule.  Worse, the files
in its working tree can be added as if they are a part of the outer
repository, which is not what the user wants.  These problems are
being addressed.

* km/empty-repo-is-still-a-repo:
add: error appropriately on repository with no commits
dir: do not traverse repositories with no commits
submodule: refuse to add repository with no commits

diff --cc dir.c
Simple merge

diff --cc git-submodule.sh
Simple merge

diff --cc read-cache.c
Simple merge