t0000: avoid masking git exit value through pipes

9af0b8dbe2 (t0000-basic: more commit-tree tests., 2006-04-26) adds
tests for commit-tree that mask the return exit from git as described
in a378fee5b0 (Documentation: add shell guidelines, 2018-10-05).

Fix the tests, to avoid pipes by using a temporary file instead.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0000-basic.sh

@@ -1271,28 +1271,29 @@ P=$(test_oid root)
 
 test_expect_success 'git commit-tree records the correct tree in a commit' '
 	commit0=$(echo NO | git commit-tree $P) &&
-	tree=$(git show --pretty=raw $commit0 |
-		 sed -n -e "s/^tree //p" -e "/^author /q") &&
+	git show --pretty=raw $commit0 >out &&
+	tree=$(sed -n -e "s/^tree //p" -e "/^author /q" out) &&
 	test "z$tree" = "z$P"
 '
 
 test_expect_success 'git commit-tree records the correct parent in a commit' '
 	commit1=$(echo NO | git commit-tree $P -p $commit0) &&
-	parent=$(git show --pretty=raw $commit1 |
-		sed -n -e "s/^parent //p" -e "/^author /q") &&
+	git show --pretty=raw $commit1 >out &&
+	parent=$(sed -n -e "s/^parent //p" -e "/^author /q" out) &&
 	test "z$commit0" = "z$parent"
 '
 
 test_expect_success 'git commit-tree omits duplicated parent in a commit' '
 	commit2=$(echo NO | git commit-tree $P -p $commit0 -p $commit0) &&
-	     parent=$(git show --pretty=raw $commit2 |
-		sed -n -e "s/^parent //p" -e "/^author /q" |
-		sort -u) &&
+	git show --pretty=raw $commit2 >out &&
+	cat >match.sed <<-\EOF &&
+	s/^parent //p
+	/^author /q
+	EOF
+	parent=$(sed -n -f match.sed out | sort -u) &&
 	test "z$commit0" = "z$parent" &&
-	numparent=$(git show --pretty=raw $commit2 |
-		sed -n -e "s/^parent //p" -e "/^author /q" |
-		wc -l) &&
-	test $numparent = 1
+	git show --pretty=raw $commit2 >out &&
+	test_stdout_line_count = 1 sed -n -f match.sed out
 '
 
 test_expect_success 'update-index D/F conflict' '