git submodule summary: Handle HEAD as argument when on an unborn branch

git submodule summary: Handle HEAD as argument when on an unborn branch

When calling "git submodule summary HEAD" on an unborn branch the output
was empty even when it shouldn't have been ("git submodule summary"
without the HEAD argument prints the expected output since commit
"submodule summary: do not fail before the first commit").

This also fixes "git status" to emit the "Submodule changes to be
committed" section on an unborn branch when used with the
status.submodulesummary config option.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>