Handle pinned posts with --fast-update and --latest-stamps (#1586)

Instead of hard-coding a check for pinned items when storing the first
item, allows the creater of NodeIterator to specify how the comparison
should be made.

This also allows storing the newest post (whatever it's status is),
instead of simply storing the newst non-pinned post, which prevents
redownloads in case a user adds only a pinned post between runs.

instaloader/nodeiterator.py

@@ -76,7 +76,8 @@ class NodeIterator(Iterator[T]):
                  node_wrapper: Callable[[Dict], T],
                  query_variables: Optional[Dict[str, Any]] = None,
                  query_referer: Optional[str] = None,
-                 first_data: Optional[Dict[str, Any]] = None):
+                 first_data: Optional[Dict[str, Any]] = None,
+                 is_first: Optional[Callable[[T], bool]] = None):
         self._context = context
         self._query_hash = query_hash
         self._edge_extractor = edge_extractor
@@ -91,6 +92,7 @@ class NodeIterator(Iterator[T]):
         else:
             self._data = self._query()
         self._first_node: Optional[Dict] = None
+        self._is_first = is_first
 
     def _query(self, after: Optional[str] = None) -> Dict:
         pagination_variables = {'first': NodeIterator._graphql_page_length}  # type: Dict[str, Any]
@@ -128,8 +130,12 @@ class NodeIterator(Iterator[T]):
                 self._page_index, self._total_index = page_index, total_index
                 raise
             item = self._node_wrapper(node)
-            if self._first_node is None:
-                self._first_node = node
+            if self._is_first is not None:
+                if self._is_first(item):
+                    self._first_node = node
+            else:
+                if self._first_node is None:
+                    self._first_node = node
             return item
         if self._data['page_info']['has_next_page']:
             query_response = self._query(self._data['page_info']['end_cursor'])
@@ -168,7 +174,13 @@ class NodeIterator(Iterator[T]):
         """
         If this iterator has produced any items, returns the first item produced.
 
+        It is possible to override what is considered the first item (for example, to consider the
+        newest item in case items are not in strict chronological order) by passing a callback
+        function as the `is_first` parameter when creating the class.
+
         .. versionadded:: 4.8
+        .. versionchanged:: 4.9.2
+           What is considered the first item can be overridden.
         """
         return self._node_wrapper(self._first_node) if self._first_node is not None else None