tests: don't lose misc "git" exit codes

Fix a few miscellaneous cases where:

- We lost the "git" exit code via "git ... | grep"
- Likewise by having a $(git) argument to git itself
- Used "test -z" to check that a command emitted no output, we can use
  "test_must_be_empty" and &&-chaining instead.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1401-symbolic-ref.sh

@@ -33,7 +33,8 @@ test_expect_success 'symbolic-ref refuses non-ref for HEAD' '
 reset_to_sane
 
 test_expect_success 'symbolic-ref refuses bare sha1' '
-	test_must_fail git symbolic-ref HEAD $(git rev-parse HEAD)
+	rev=$(git rev-parse HEAD) &&
+	test_must_fail git symbolic-ref HEAD "$rev"
 '
 
 reset_to_sane

t/t3701-add-interactive.sh

@@ -296,9 +296,11 @@ test_expect_success FILEMODE 'stage mode and hunk' '
 	echo content >>file &&
 	chmod +x file &&
 	printf "y\\ny\\n" | git add -p &&
-	git diff --cached file | grep "new mode" &&
-	git diff --cached file | grep "+content" &&
-	test -z "$(git diff file)"
+	git diff --cached file >out &&
+	grep "new mode" out &&
+	grep "+content" out &&
+	git diff file >out &&
+	test_must_be_empty out
 '
 
 # end of tests disabled when filemode is not usable

t/t7516-commit-races.sh

@@ -10,7 +10,8 @@ test_expect_success 'race to create orphan commit' '
 	test_must_fail env EDITOR=./hare-editor git commit --allow-empty -m tortoise -e &&
 	git show -s --pretty=format:%s >subject &&
 	grep hare subject &&
-	test -z "$(git show -s --pretty=format:%P)"
+	git show -s --pretty=format:%P >out &&
+	test_must_be_empty out
 '
 
 test_expect_success 'race to create non-orphan commit' '