fileset: don't suggest private or undocumented queries

fileset: don't suggest private or undocumented queries

Unlike revsets, it looks like all of the filesets are documented, so there's
really nothing to test.  This is aimed more at parity with revsets and
future-proofing.

--- a/mercurial/fileset.py	Sat Jun 20 10:59:56 2015 -0400
+++ b/mercurial/fileset.py	Sat Jun 20 11:07:36 2015 -0400
@@ -189,7 +189,11 @@
 def func(mctx, a, b):
     if a[0] == 'symbol' and a[1] in symbols:
         return symbols[a[1]](mctx, b)
-    raise error.UnknownIdentifier(a[1], symbols.keys())
+
+    keep = lambda fn: getattr(fn, '__doc__', None) is not None
+
+    syms = [s for (s, fn) in symbols.items() if keep(fn)]
+    raise error.UnknownIdentifier(a[1], syms)
 
 def getlist(x):
     if not x: