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>

git-submodule.sh

@@ -553,12 +553,15 @@ cmd_summary() {
 
 	test $summary_limit = 0 && return
 
-	if rev=$(git rev-parse -q --verify "$1^0")
+	if rev=$(git rev-parse -q --verify --default HEAD ${1+"$1"})
 	then
 		head=$rev
 		shift
+	elif test -z "$1" -o "$1" = "HEAD"
+	then
+		return
 	else
-		head=HEAD
+		head="HEAD"
 	fi
 
 	if [ -n "$files" ]

t/t7401-submodule-summary.sh

@@ -227,4 +227,11 @@ test_expect_success 'fail when using --files together with --cached' "
     test_must_fail git submodule summary --files --cached
 "
 
+test_expect_success 'should not fail in an empty repo' "
+    git init xyzzy &&
+    cd xyzzy &&
+    git submodule summary >output 2>&1 &&
+    test_cmp output /dev/null
+"
+
 test_done