revset: don't suggest private or undocumented queries

I noticed when I mistyped 'matching', that it suggested '_matchfiles' as well.
Rather than simply exclude names that start with '_', this excludes anything
without a docstring.  That way, if it isn't in the help text, it isn't
suggested, such as 'wdir()'.

mercurial/revset.py

@@ -391,7 +391,11 b' def listset(repo, subset, a, b):'
 def func(repo, subset, a, b):
     if a[0] == 'symbol' and a[1] in symbols:
         return symbols[a[1]](repo, subset, 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)
 
 # functions
 

tests/test-revset.t

@@ -1334,6 +1334,17 b' Bogus function gets suggestions'
   hg: parse error: unknown identifier: babar
   [255]
 
+Bogus function with a similar internal name doesn't suggest the internal name
+  $ log 'matches()'
+  hg: parse error: unknown identifier: matches
+  (did you mean 'matching'?)
+  [255]
+
+Undocumented functions aren't suggested as similar either
+  $ log 'wdir2()'
+  hg: parse error: unknown identifier: wdir2
+  [255]
+
 multiple revspecs
 
   $ hg log -r 'tip~1:tip' -r 'tip~2:tip~1' --template '{rev}\n'