revlog: replace flag check related to generaldelta with attribute check

Same logic as the previous changesets.

Differential Revision: https://phab.mercurial-scm.org/D10563

mercurial/debugcommands.py

@@ -793,7 +793,7 b' def debugdeltachain(ui, repo, file_=None'
     index = r.index
     start = r.start
     length = r.length
-    generaldelta = r.version & revlog.FLAG_GENERALDELTA
+    generaldelta = r._generaldelta
     withsparseread = getattr(r, '_withsparseread', False)
 
     def revinfo(rev):