t0090: avoid passing empty string to printf %d

FreeBSD's printf(1) doesn't accept empty strings for numerical format
specifiers:

	$ printf "%d\n" "" >/dev/null; echo $?
	printf: : expected numeric value
	1

Initialize the AWK variable c to make sure the shell variable
subtree_count always contains a numerical value, in order to keep the
subsequently called printf happy.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0090-cache-tree.sh

@@ -22,7 +22,7 @@ generate_expected_cache_tree_rec () {
 	# ls-files might have foo/bar, foo/bar/baz, and foo/bar/quux
 	# We want to count only foo because it's the only direct child
 	subtrees=$(git ls-files|grep /|cut -d / -f 1|uniq) &&
-	subtree_count=$(echo "$subtrees"|awk '$1 {++c} END {print c}') &&
+	subtree_count=$(echo "$subtrees"|awk -v c=0 '$1 {++c} END {print c}') &&
 	entries=$(git ls-files|wc -l) &&
 	printf "SHA $dir (%d entries, %d subtrees)\n" "$entries" "$subtree_count" &&
 	for subtree in $subtrees