t5520: 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/t5520-pull.sh

@@ -96,8 +96,7 @@ test_expect_success '--rebase' '
 '
 test_expect_success 'pull.rebase' '
 	git reset --hard before-rebase &&
-	git config --bool pull.rebase true &&
-	test_when_finished "git config --unset pull.rebase" &&
+	test_config pull.rebase true &&
 	git pull . copy &&
 	test $(git rev-parse HEAD^) = $(git rev-parse copy) &&
 	test new = $(git show HEAD:file2)
@@ -105,8 +104,7 @@ test_expect_success 'pull.rebase' '
 
 test_expect_success 'branch.to-rebase.rebase' '
 	git reset --hard before-rebase &&
-	git config --bool branch.to-rebase.rebase true &&
-	test_when_finished "git config --unset branch.to-rebase.rebase" &&
+	test_config branch.to-rebase.rebase true &&
 	git pull . copy &&
 	test $(git rev-parse HEAD^) = $(git rev-parse copy) &&
 	test new = $(git show HEAD:file2)
@@ -114,10 +112,8 @@ test_expect_success 'branch.to-rebase.rebase' '
 
 test_expect_success 'branch.to-rebase.rebase should override pull.rebase' '
 	git reset --hard before-rebase &&
-	git config --bool pull.rebase true &&
-	test_when_finished "git config --unset pull.rebase" &&
-	git config --bool branch.to-rebase.rebase false &&
-	test_when_finished "git config --unset branch.to-rebase.rebase" &&
+	test_config pull.rebase true &&
+	test_config branch.to-rebase.rebase false &&
 	git pull . copy &&
 	test $(git rev-parse HEAD^) != $(git rev-parse copy) &&
 	test new = $(git show HEAD:file2)