diff --git a/t/t5801-remote-helpers.sh b/t/t5801-remote-helpers.sh
index c9d3ed14c3..362b1581e0 100755
--- a/t/t5801-remote-helpers.sh
+++ b/t/t5801-remote-helpers.sh
@@ -242,13 +242,6 @@ clean_mark () {
 	sort >$(basename "$1")
 }
 
-cmp_marks () {
-	test_when_finished "rm -rf git.marks testgit.marks" &&
-	clean_mark ".git/testgit/$1/git.marks" &&
-	clean_mark ".git/testgit/$1/testgit.marks" &&
-	test_cmp git.marks testgit.marks
-}
-
 test_expect_success 'proper failure checks for fetching' '
 	(cd local &&
 	test_must_fail env GIT_REMOTE_TESTGIT_FAILURE=1 git fetch 2>error &&
@@ -258,12 +251,15 @@ test_expect_success 'proper failure checks for fetching' '
 '
 
 test_expect_success 'proper failure checks for pushing' '
+	test_when_finished "rm -rf local/git.marks local/testgit.marks" &&
 	(cd local &&
 	git checkout -b crash master &&
 	echo crash >>file &&
 	git commit -a -m crash &&
 	test_must_fail env GIT_REMOTE_TESTGIT_FAILURE=1 git push --all &&
-	cmp_marks origin
+	clean_mark ".git/testgit/origin/git.marks" &&
+	clean_mark ".git/testgit/origin/testgit.marks" &&
+	test_cmp git.marks testgit.marks
 	)
 '
 
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index 7eeb207b32..6f12b235b3 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -174,9 +174,9 @@ test_expect_success 'mergetool skips autoresolved' '
 '
 
 test_expect_success 'mergetool merges all from subdir' '
+	test_config rerere.enabled false &&
 	(
 		cd subdir &&
-		test_config rerere.enabled false &&
 		test_must_fail git merge master &&
 		( yes "r" | git mergetool ../submod ) &&
 		( yes "d" "d" | git mergetool --no-prompt ) &&
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index a771cf77f5..ec8bc8c765 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -492,12 +492,12 @@ test_expect_success PERL 'difftool --no-symlinks detects conflict ' '
 
 test_expect_success PERL 'difftool properly honors gitlink and core.worktree' '
 	git submodule add ./. submod/ule &&
+	test_config -C submod/ule diff.tool checktrees &&
+	test_config -C submod/ule difftool.checktrees.cmd '\''
+		test -d "$LOCAL" && test -d "$REMOTE" && echo good
+		'\'' &&
 	(
 		cd submod/ule &&
-		test_config diff.tool checktrees &&
-		test_config difftool.checktrees.cmd '\''
-			test -d "$LOCAL" && test -d "$REMOTE" && echo good
-		'\'' &&
 		echo good >expect &&
 		git difftool --tool=checktrees --dir-diff HEAD~ >actual &&
 		test_cmp expect actual
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index e8d3c0fdbc..6dffb8bcde 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -201,7 +201,14 @@ test_chmod () {
 
 # Unset a configuration variable, but don't fail if it doesn't exist.
 test_unconfig () {
-	git config --unset-all "$@"
+	config_dir=
+	if test "$1" = -C
+	then
+		shift
+		config_dir=$1
+		shift
+	fi
+	git ${config_dir:+-C "$config_dir"} config --unset-all "$@"
 	config_status=$?
 	case "$config_status" in
 	5) # ok, nothing to unset
@@ -213,8 +220,15 @@ test_unconfig () {
 
 # Set git config, automatically unsetting it after the test is over.
 test_config () {
-	test_when_finished "test_unconfig '$1'" &&
-	git config "$@"
+	config_dir=
+	if test "$1" = -C
+	then
+		shift
+		config_dir=$1
+		shift
+	fi
+	test_when_finished "test_unconfig ${config_dir:+-C '$config_dir'} '$1'" &&
+	git ${config_dir:+-C "$config_dir"} config "$@"
 }
 
 test_config_global () {
@@ -722,6 +736,11 @@ test_seq () {
 # what went wrong.
 
 test_when_finished () {
+	# We cannot detect when we are in a subshell in general, but by
+	# doing so on Bash is better than nothing (the test will
+	# silently pass on other shells).
+	test "${BASH_SUBSHELL-0}" = 0 ||
+	error "bug in test script: test_when_finished does nothing in a subshell"
 	test_cleanup="{ $*
 		} && (exit \"\$eval_ret\"); eval_ret=\$?; $test_cleanup"
 }