git-submodule.sh: shorten submodule SHA-1s using rev-parse

git-submodule.sh: shorten submodule SHA-1s using rev-parse

Until now, `git submodule summary` was always emitting 7-character
SHA-1s that have a higher chance of being ambiguous for larger
repositories.  Use `git rev-parse --short` instead, which will
determine suitable short SHA-1 lengths.

When a submodule hasn't been initialized with "submodule init" or
not cloned, `git rev-parse` would not work in it yet; as a fallback,
use the original method of cutting at 7 hexdigits.

Signed-off-by: Sven van Haastregt <svenvh@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>