submodule summary: do not fail before the first commit

submodule summary: do not fail before the first commit

When "git status" collects changes for the index (usually relative to
HEAD), it compares the index with an empty tree when the repository does
not have an initial commit yet.  "git submodule summary" is about asking
what submodule changes would be recorded if a commit is made right now,
and should do the same comparison to report all the added submodules,
instead of punting and being silent.

Signed-off-by: Junio C Hamano <gitster@pobox.com>