doctest: add a `issnapshot` method to _testrevlog

We'll need it soon.

mercurial/revlogutils/deltas.py

@@ -38,11 +38,12 b' LIMIT_DELTA2TEXT = 2'
 class _testrevlog(object):
     """minimalist fake revlog to use in doctests"""
 
-    def __init__(self, data, density=0.5, mingap=0):
+    def __init__(self, data, density=0.5, mingap=0, snapshot=()):
         """data is an list of revision payload boundaries"""
         self._data = data
         self._srdensitythreshold = density
         self._srmingapsize = mingap
+        self._snapshot = set(snapshot)
 
     def start(self, rev):
         if rev == 0:
@@ -58,6 +59,9 b' class _testrevlog(object):'
     def __len__(self):
         return len(self._data)
 
+    def issnapshot(self, rev):
+        return rev in self._snapshot
+
 def slicechunk(revlog, revs, targetsize=None):
     """slice revs to reduce the amount of unrelated data to be read from disk.