emitrevision: add a small closure to check if a base is usable

We will make more use of this and make it more complex too.

mercurial/utils/storageutil.py

@@ -396,6 +396,9 b' def emitrevisions('
     # Set of revs available to delta against.
     available = set()
 
+    def is_usable_base(rev):
+        return rev != nullrev and rev in available
+
     for rev in revs:
         if rev == nullrev:
             continue
@@ -445,7 +448,7 b' def emitrevisions('
         elif deltaparentrev != nullrev:
             # Base revision was already emitted in this group. We can
             # always safely use the delta.
-            if deltaparentrev in available:
+            if is_usable_base(deltaparentrev):
                 if debug_info is not None:
                     debug_delta_source = "storage"
                 baserev = deltaparentrev