test-lib-functions: support "test_config -C <dir> ..."

If used in a subshell, test_config cannot unset variables at the end of
a test.  This is a problem when testing submodules because we do not
want to "cd" at to top level of a test script in order to run the
command inside the submodule.

Add a "-C" option to test_config (and test_unconfig) so that test_config
can be kept outside subshells and still affect subrepositories.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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 () {