diff -r 5cb72229f0e9 -r 1b183edbb68e mercurial/repository.py
--- a/mercurial/repository.py	Fri Oct 19 13:44:25 2018 +0200
+++ b/mercurial/repository.py	Wed Sep 26 14:41:15 2018 -0700
@@ -693,7 +693,8 @@
         applying raw data from a peer repo.
         """
 
-    def addgroup(deltas, linkmapper, transaction, addrevisioncb=None):
+    def addgroup(deltas, linkmapper, transaction, addrevisioncb=None,
+                 maybemissingparents=False):
         """Process a series of deltas for storage.
 
         ``deltas`` is an iterable of 7-tuples of
@@ -707,6 +708,11 @@
 
         ``addrevisioncb`` should be called for each node as it is committed.
 
+        ``maybemissingparents`` is a bool indicating whether the incoming
+        data may reference parents/ancestor revisions that aren't present.
+        This flag is set when receiving data into a "shallow" store that
+        doesn't hold all history.
+
         Returns a list of nodes that were processed. A node will be in the list
         even if it existed in the store previously.
         """