t0090: stop losing return codes of git commands

In generate_expected_cache_tree_rec(), there are currently two instances
of `git ls-files` in the upstream of a pipe. In the case where the
upstream git command fails, its return code will be lost. Extract the
`git ls-files` into its own call so that if it ever fails, its return
code is not lost.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0090-cache-tree.sh

@@ -21,9 +21,10 @@ generate_expected_cache_tree_rec () {
 	parent="$2" &&
 	# ls-files might have foo/bar, foo/bar/baz, and foo/bar/quux
 	# We want to count only foo because it's the only direct child
-	subtrees=$(git ls-files|grep /|cut -d / -f 1|uniq) &&
+	git ls-files >files &&
+	subtrees=$(grep / files|cut -d / -f 1|uniq) &&
 	subtree_count=$(echo "$subtrees"|awk -v c=0 '$1 != "" {++c} END {print c}') &&
-	entries=$(git ls-files|wc -l) &&
+	entries=$(wc -l <files) &&
 	printf "SHA $dir (%d entries, %d subtrees)\n" "$entries" "$subtree_count" &&
 	for subtree in $subtrees
 	do