Add test config parsing

This tests parsing a multiline string containing multiple quoted comment
chars. See #6019

tests/config/read.c

@@ -213,6 +213,13 @@ void test_config_read__symbol_headers(void)
 	git_config_free(cfg);
 }
 
+void test_config_read__multiline_multiple_quoted_comment_chars(void)
+{
+	git_config *cfg;
+	cl_git_pass(git_config_open_ondisk(&cfg, cl_fixture("config/config21")));
+	git_config_free(cfg);
+}
+
 void test_config_read__header_in_last_line(void)
 {
 	git_config *cfg;

tests/resources/config/config21

+[alias]
+	m = '\
+	";" \
+	";" \
+	'