fileset: handle underbar in symbols stable

fileset: handle underbar in symbols

This was documented, but not implemented.

--- a/mercurial/fileset.py	Sun Jul 21 18:45:42 2013 -0500
+++ b/mercurial/fileset.py	Tue Jul 23 14:18:56 2013 -0500
@@ -27,7 +27,7 @@
 
 keywords = set(['and', 'or', 'not'])
 
-globchars = ".*{}[]?/\\"
+globchars = ".*{}[]?/\\_"
 
 def tokenize(program):
     pos, l = 0, len(program)

--- a/tests/test-fileset.t	Sun Jul 21 18:45:42 2013 -0500
+++ b/tests/test-fileset.t	Tue Jul 23 14:18:56 2013 -0500
@@ -42,6 +42,7 @@
   b2
   $ fileset 'a* - a1'
   a2
+  $ fileset 'a_b'
 
 Test files status