config: borrow the string for lookup and setting

We don't need our own copy of the string, so use the new borrowing
mechanism to use python's underlying string for the key to get/set.

src/config.c

@@ -147,15 +147,15 @@ Config_get_system_config(void)
 int
 Config_contains(Config *self, PyObject *py_key) {
     int err;
-    const char *c_value;
+    const char *c_value, *c_key;
-    char *c_key;
+    PyObject *tkey;
 
-    c_key = py_str_to_c_str(py_key, NULL);
+    c_key = py_str_borrow_c_str(&tkey, py_key, NULL);
     if (c_key == NULL)
         return -1;
 
     err = git_config_get_string(&c_value, self->config, c_key);
-    free(c_key);
+    Py_DECREF(tkey);
 
     if (err < 0) {
         if (err == GIT_ENOTFOUND)
@@ -175,14 +175,15 @@ Config_getitem(Config *self, PyObject *py_key)
     int64_t value_int;
     int err, value_bool;
     const char *value_str;
-    char *key;
+    const char *key;
-    PyObject* py_value;
+    PyObject* py_value, *tmp;
 
-    key = py_str_to_c_str(py_key, NULL);
+    key = py_str_borrow_c_str(&tmp, py_key, NULL);
     if (key == NULL)
         return NULL;
 
     err = git_config_get_string(&value_str, self->config, key);
+    Py_CLEAR(tmp);
     if (err < 0)
         goto cleanup;
 
@@ -194,8 +195,6 @@ Config_getitem(Config *self, PyObject *py_key)
         py_value = to_unicode(value_str, NULL, NULL);
 
 cleanup:
-    free(key);
-
     if (err < 0) {
         if (err == GIT_ENOTFOUND) {
             PyErr_SetObject(PyExc_KeyError, py_key);
@@ -212,9 +211,10 @@ int
 Config_setitem(Config *self, PyObject *py_key, PyObject *py_value)
 {
     int err;
-    char *key, *value;
+    const char *key, *value;
+    PyObject *tkey, *tvalue;
 
-    key = py_str_to_c_str(py_key, NULL);
+    key = py_str_borrow_c_str(&tkey, py_key, NULL);
     if (key == NULL)
         return -1;
 
@@ -227,12 +227,12 @@ Config_setitem(Config *self, PyObject *py_key, PyObject *py_value)
         err = git_config_set_int64(self->config, key,
                 (int64_t)PyLong_AsLong(py_value));
     } else {
-        value = py_str_to_c_str(py_value, NULL);
+        value = py_str_borrow_c_str(&tvalue, py_value, NULL);
         err = git_config_set_string(self->config, key, value);
-        free(value);
+        Py_DECREF(tvalue);
     }
 
-    free(key);
+    Py_DECREF(tkey);
     if (err < 0) {
         Error_set(err);
         return -1;