submodule summary: Don't barf when invoked in an empty repo

submodule summary: Don't barf when invoked in an empty repo

When invoking "git submodule summary" in an empty repo (which can be
indirectly done by setting status.submodulesummary = true), it currently
emits an error message (via "git diff-index") since HEAD points to an
unborn branch.

This patch adds handling of the HEAD-points-to-unborn-branch special case,
so that "git submodule summary" no longer emits this error message.

The patch also adds a test case that verifies the fix.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>