[PATCH] config: avoid using a mutable default

[Martijn Pieters]

# HG changeset patch
# User Martijn Pieters <mjpieters at fb.com>
# Date 1489348572 25200
#      Sun Mar 12 12:56:12 2017 -0700
# Node ID 22c38e571b5ccf3bb6d9f075526170954843f37a
# Parent  719e64bf9ec2d7b8e86b6550a5d193b3c67944d1
config: avoid using a mutable default

Nothing *currently* mutates this list, but the moment something does it'll be
shared between all config instances. Avoid this eventuality.

diff --git a/mercurial/config.py b/mercurial/config.py
--- a/mercurial/config.py
+++ b/mercurial/config.py
@@ -18,11 +18,11 @@
 )
 
 class config(object):
-    def __init__(self, data=None, includepaths=[]):
+    def __init__(self, data=None, includepaths=None):
         self._data = {}
         self._source = {}
         self._unset = []
-        self._includepaths = includepaths
+        self._includepaths = includepaths or []
         if data:
             for k in data._data:
                 self._data[k] = data[k].copy()