revset: don't error out if tokens parse as existing symbols

It makes perfect sense for tokens to parse as existing revset symbols
(revset functions), and doesn't break anything, since parsing symbols
as functions works correctly in the presence of parens. For example,
if "only" is a bookmark, this used to error out,

   hg log -r "only(only, @)"

which shouldn't, as the inner "only" is unambiguously not a function.

So we just remove the symbolset function and replace its calling site
with the stringset function.

For the tests, we confirm that "date" and "only" are both parsed as
revision names both inside revset expressions (e.g. an expression
containing ::) and inside old-style revision expressions (e.g. those
containing the name of the revision alone).

mercurial/revset.py

@@ -334,11 +334,6 b' def stringset(repo, subset, x):'
         return baseset([x])
     return baseset()
 
-def symbolset(repo, subset, x):
-    if x in symbols:
-        raise error.ParseError(_("can't use %s here") % x)
-    return stringset(repo, subset, x)
-
 def rangeset(repo, subset, x, y):
     m = getset(repo, fullreposet(repo), x)
     n = getset(repo, fullreposet(repo), y)
@@ -2088,7 +2083,7 b' methods = {'
     "range": rangeset,
     "dagrange": dagrange,
     "string": stringset,
-    "symbol": symbolset,
+    "symbol": stringset,
     "and": andset,
     "or": orset,
     "not": notset,

tests/test-revset.t

@@ -281,7 +281,7 b' quoting needed'
   hg: parse error: date requires a string
   [255]
   $ log 'date'
-  hg: parse error: can't use date here
+  abort: unknown revision 'date'!
   [255]
   $ log 'date('
   hg: parse error at 5: not a prefix: end
@@ -289,11 +289,40 b' quoting needed'
   $ log 'date(tip)'
   abort: invalid date: 'tip'
   [255]
-  $ log '"date"'
+  $ log '0:date'
+  abort: unknown revision 'date'!
+  [255]
+  $ log '::"date"'
   abort: unknown revision 'date'!
   [255]
+  $ hg book date -r 4
+  $ log '0:date'
+  0
+  1
+  2
+  3
+  4
+  $ log '::date'
+  0
+  1
+  2
+  4
+  $ log '::"date"'
+  0
+  1
+  2
+  4
   $ log 'date(2005) and 1::'
   4
+  $ hg book -d date
+
+Test that symbols only get parsed as functions if there's an opening
+parenthesis.
+
+  $ hg book only -r 9
+  $ log 'only(only)'   # Outer "only" is a function, inner "only" is the bookmark
+  8
+  9
 
 ancestor can accept 0 or more arguments