diff -r 6a917075535a -r 968dd7e02ac5 mercurial/utils/storageutil.py
--- a/mercurial/utils/storageutil.py	Tue Oct 09 23:26:35 2018 +0200
+++ b/mercurial/utils/storageutil.py	Wed Oct 10 00:21:02 2018 +0200
@@ -373,6 +373,9 @@
         # We're instructed to send fulltext. Honor that.
         elif deltamode == repository.CG_DELTAMODE_FULL:
             baserev = nullrev
+        # We're instructed to use p1. Honor that
+        elif deltamode == repository.CG_DELTAMODE_P1:
+            baserev = p1rev
 
         # There is a delta in storage. We try to use that because it
         # amounts to effectively copying data from storage and is