[PATCH remotefilelog-ext remotefilelog-ext] datapack: don't depend on demandimport when cstore isn't available

[Augie Fackler]

# HG changeset patch
# User Augie Fackler <augie at google.com>
# Date 1489431594 14400
#      Mon Mar 13 14:59:54 2017 -0400
# Node ID 7d290452e8f119b1329f70c9d529a8c5ff271cea
# Parent  dd7bb6e663c80553a6abb197bbe74de439c4410f
datapack: don't depend on demandimport when cstore isn't available

We've got a goofy test binary that doesn't use demandimport, and this
was tripping it up.

diff --git a/remotefilelog/datapack.py b/remotefilelog/datapack.py
--- a/remotefilelog/datapack.py
+++ b/remotefilelog/datapack.py
@@ -3,7 +3,11 @@ from mercurial import util
 from mercurial.node import nullid, hex
 from mercurial.i18n import _
 import basepack, constants
-import cstore
+try:
+    import cstore
+    cstore.datapack
+except ImportError:
+    cstore = None
 
 # Index entry format is: <node><delta offset><pack data offset><pack data size>
 # See the mutabledatapack doccomment for more details.