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

git-submodule.sh

@@ -850,8 +850,11 @@ cmd_summary() {
 			;;
 		esac
 
-		sha1_abbr_src=$(echo $sha1_src | cut -c1-7)
-		sha1_abbr_dst=$(echo $sha1_dst | cut -c1-7)
+		sha1_abbr_src=$(GIT_DIR="$name/.git" git rev-parse --short $sha1_src 2>/dev/null ||
+			echo $sha1_src | cut -c1-7)
+		sha1_abbr_dst=$(GIT_DIR="$name/.git" git rev-parse --short $sha1_dst 2>/dev/null ||
+			echo $sha1_dst | cut -c1-7)
+
 		if test $status = T
 		then
 			blob="$(gettext "blob")"