revlog: add a method to tells whether rev is stored as a snapshot

For now we only have one type of snapshot: full snapshot versus nullrev.
However we are looking into adding intermediate snapshot where a large diff
against another snapshot is performed instead of storing a full new text.

The conditional is a bit strange and is done in order to help readability of a
some later changesets.

mercurial/revlog.py

@@ -2095,6 +2095,16 b' class revlog(object):'
         else:
             return rev - 1
 
+    def issnapshot(self, rev):
+        """tells whether rev is a snapshot
+        """
+        if rev == nullrev:
+            return True
+        deltap = self.deltaparent(rev)
+        if deltap == nullrev:
+            return True
+        return False
+
     def revdiff(self, rev1, rev2):
         """return or calculate a delta between two revisions