config: add failing test for an external modification

We currently use the timestamp in order to decide whether a config file
has changed since we last read it.

This scheme falls down if the file is written twice within the same
second, as we fail to detect the file change after the first read in
that second.

tests/config/stress.c

@@ -107,3 +107,23 @@ void test_config_stress__complex(void)
 
 	git_config_free(config);
 }
+
+void test_config_stress__quick_write(void)
+{
+	git_config *config_w, *config_r;
+	const char *path = "./config-quick-write";
+	const char *key = "quick.write";
+	int32_t i;
+
+	/* Create an external writer for one instance with the other one */
+	cl_git_pass(git_config_open_ondisk(&config_w, path));
+	cl_git_pass(git_config_open_ondisk(&config_r, path));
+
+	/* Write and read in the same second (repeat to increase the chance of it happening) */
+	for (i = 0; i < 10; i++) {
+		int32_t val;
+		cl_git_pass(git_config_set_int32(config_w, key, i));
+		cl_git_pass(git_config_get_int32(&val, config_r, key));
+		cl_assert_equal_i(i, val);
+	}
+}