test-setdiscovery: allow for leading space in output of wc

tests/test-setdiscovery.t

@@ -288,7 +288,7 b' One with >200 heads, which used to use u'
   reading DAG from stdin
 
   $ hg heads -t --template . | wc -c
-  261
+  \s*261 (re)
 
   $ hg clone -b a . a
   adding changesets