submodule: make zero-oid comparison hash function agnostic

With SHA-256, the length of the all-zeros object ID is longer.  Add a
function to git-submodule.sh to check if a full hex object ID is the
all-zeros value, and use it to check the output we're parsing from git
diff-files or diff-index.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -82,6 +82,11 @@ isnumber()
 	n=$(($1 + 0)) 2>/dev/null && test "$n" = "$1"
 }
 
+# Given a full hex object ID, is this the zero OID?
+is_zero_oid () {
+	echo "$1" | sane_egrep '^0+$' >/dev/null 2>&1
+}
+
 # Sanitize the local git environment for use within a submodule. We
 # can't simply use clear_local_git_env since we want to preserve some
 # of the settings from GIT_CONFIG_PARAMETERS.
@@ -780,7 +785,7 @@ cmd_summary() {
 	while read -r mod_src mod_dst sha1_src sha1_dst status name
 	do
 		if test -z "$cached" &&
-			test $sha1_dst = 0000000000000000000000000000000000000000
+			is_zero_oid $sha1_dst
 		then
 			case "$mod_dst" in
 			160000)