When working in the top-level project, it is useful to create a new
submodule as a git repo in a subdirectory, then add that submodule to
the top-level in place.

This patch allows "git submodule add <intended url> subdir" to add the
existing subdir to the current project.  The presumption is the user will
later push / clone the subdir to the <intended url> so that future
submodule init / updates will work.

Absent this patch, "git submodule add" insists upon cloning the subdir
from a repository at the given url, which is fine for adding an existing
project in, but less useful when adding a new submodule from scratch to an
existing project.  The former functionality remains, and the clone is
attempted if the subdir does not already exist as a valid git repo.
Signed-off-by: NMark Levedahl <mlevedahl@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>