exchange: fix an attempt to format a list into bytes

exchange: fix an attempt to format a list into bytes

This goes with 27c6d6f53d46.

Differential Revision: https://phab.mercurial-scm.org/D7583

--- a/mercurial/exchange.py	Mon Dec 09 09:50:39 2019 -0800
+++ b/mercurial/exchange.py	Sat Dec 07 22:09:43 2019 -0500
@@ -2202,7 +2202,7 @@
     if invalid_includes:
         raise error.Abort(
             _(b"The following includes are not accessible for %s: %s")
-            % (username, invalid_includes)
+            % (username, stringutil.pprint(invalid_includes))
         )
 
     new_args = {}