deltaparent(): don't return nullrev for a revision containing a full snapshot

this allows us to simplify manifest.readdelta and revlog.revdiff

mercurial/manifest.py

@@ -36,9 +36,7 b' class manifest(revlog.revlog):'
 
     def readdelta(self, node):
         r = self.rev(node)
-        if self._parentdelta:
-            return self.parse(mdiff.patchtext(self.revdiff(self.deltaparent(r), r)))
-        return self.parse(mdiff.patchtext(self.revdiff(r - 1, r)))
+        return self.parse(mdiff.patchtext(self.revdiff(self.deltaparent(r), r)))
 
     def read(self, node):
         if node == revlog.nullid:

mercurial/revlog.py

@@ -1018,16 +1018,14 b' class revlog(object):'
 
     def deltaparent(self, rev):
         """return previous revision or parentrev according to flags"""
-        if self.base(rev) == rev:
-            return nullrev
-        elif self.flags(rev) & REVIDX_PARENTDELTA:
+        if self.flags(rev) & REVIDX_PARENTDELTA:
             return self.parentrevs(rev)[0]
         else:
             return rev - 1
 
     def revdiff(self, rev1, rev2):
         """return or calculate a delta between two revisions"""
-        if rev1 != nullrev and self.deltaparent(rev2) == rev1:
+        if self.base(rev2) != rev2 and self.deltaparent(rev2) == rev1:
             return self._chunk(rev2)
 
         return mdiff.textdiff(self.revision(self.node(rev1)),