revlog: add a method to retrieve snapshot depth

Some snapshot property (eg: maximum size) will depend on their depth.

mercurial/revlog.py

@@ -2108,6 +2108,12 b' class revlog(object):'
             return False
         return self.issnapshot(deltap)
 
+    def snapshotdepth(self, rev):
+        """number of snapshot in the chain before this one"""
+        if not self.issnapshot(rev):
+            raise ProgrammingError('revision %d not a snapshot')
+        return len(self._deltachain(rev)[0]) - 1
+
     def revdiff(self, rev1, rev2):
         """return or calculate a delta between two revisions