submodule: refuse to add repository with no commits

When the path given to 'git submodule add' is an existing repository
that is not in the index, the repository is passed to 'git add'.  If
this repository doesn't have a commit checked out, we don't get a
useful result: there is no subproject OID to track, and any untracked
files in the sub-repository are added as blobs in the top-level
repository.

To avoid getting into this state, abort if the path is a repository
that doesn't have a commit checked out.  Note that this check must
come before the 'git add --dry-run' check because the next commit will
make 'git add' fail when given a repository that doesn't have a commit
checked out.

Signed-off-by: Kyle Meyer <kyle@kyleam.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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