t7500: use test_config to set/unset git config variables

Instead of using construct such as:
    test_when_finished "git config --unset <key>"
    git config <key> <value>
uses
    test_config <key> <value>
The latter takes care of removing <key> at the end of the test.

Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7500-commit.sh

@@ -36,8 +36,7 @@ test_expect_success 'nonexistent template file should return error' '
 '
 
 test_expect_success 'nonexistent template file in config should return error' '
-	git config commit.template "$PWD"/notexist &&
-	test_when_finished "git config --unset commit.template" &&
+	test_config commit.template "$PWD"/notexist &&
 	(
 		GIT_EDITOR="echo hello >\"\$1\"" &&
 		export GIT_EDITOR &&
@@ -93,14 +92,13 @@ test_expect_success '-t option should be short for --template' '
 
 test_expect_success 'config-specified template should commit' '
 	echo "new template" > "$TEMPLATE" &&
-	git config commit.template "$TEMPLATE" &&
+	test_config commit.template "$TEMPLATE" &&
 	echo "more content" >> foo &&
 	git add foo &&
 	(
 		test_set_editor "$TEST_DIRECTORY"/t7500/add-content &&
 		git commit
 	) &&
-	git config --unset commit.template &&
 	commit_msg_is "new templatecommit message"
 '