help: avoid using "$n" parameter in revsetalias example

Because parsing "$n" requires a crafted tokenizer, it exists only for backward
compatibility (as documented in revset._tokenizealias.) This patch updates the
examples so that users are encouraged to use symbolic names instead of "$n"s.

I'm going to implement alias expansion in templater, which won't support "$n"
parameters to make my life easier. Templater is more complicated than revset
because tokenizer and parser call each other.

mercurial/help/revsets.txt

@@ -1,134 +1,134 b''
 Mercurial supports a functional language for selecting a set of
 revisions.
 
 The language supports a number of predicates which are joined by infix
 operators. Parenthesis can be used for grouping.
 
 Identifiers such as branch names may need quoting with single or
 double quotes if they contain characters like ``-`` or if they match
 one of the predefined predicates.
 
 Special characters can be used in quoted identifiers by escaping them,
 e.g., ``\n`` is interpreted as a newline. To prevent them from being
 interpreted, strings can be prefixed with ``r``, e.g. ``r'...'``.
 
 There is a single prefix operator:
 
 ``not x``
   Changesets not in x. Short form is ``! x``.
 
 These are the supported infix operators:
 
 ``x::y``
   A DAG range, meaning all changesets that are descendants of x and
   ancestors of y, including x and y themselves. If the first endpoint
   is left out, this is equivalent to ``ancestors(y)``, if the second
   is left out it is equivalent to ``descendants(x)``.
 
   An alternative syntax is ``x..y``.
 
 ``x:y``
   All changesets with revision numbers between x and y, both
   inclusive. Either endpoint can be left out, they default to 0 and
   tip.
 
 ``x and y``
   The intersection of changesets in x and y. Short form is ``x & y``.
 
 ``x or y``
   The union of changesets in x and y. There are two alternative short
   forms: ``x | y`` and ``x + y``.
 
 ``x - y``
   Changesets in x but not in y.
 
 ``x^n``
   The nth parent of x, n == 0, 1, or 2.
   For n == 0, x; for n == 1, the first parent of each changeset in x;
   for n == 2, the second parent of changeset in x.
 
 ``x~n``
   The nth first ancestor of x; ``x~0`` is x; ``x~3`` is ``x^^^``.
 
 There is a single postfix operator:
 
 ``x^``
   Equivalent to ``x^1``, the first parent of each changeset in x.
 
 
 The following predicates are supported:
 
 .. predicatesmarker
 
 New predicates (known as "aliases") can be defined, using any combination of
 existing predicates or other aliases. An alias definition looks like::
 
   <alias> = <definition>
 
 in the ``revsetalias`` section of a Mercurial configuration file. Arguments
-of the form `$1`, `$2`, etc. are substituted from the alias into the
+of the form `a1`, `a2`, etc. are substituted from the alias into the
 definition.
 
 For example,
 
 ::
 
   [revsetalias]
   h = heads()
-  d($1) = sort($1, date)
-  rs($1, $2) = reverse(sort($1, $2))
+  d(s) = sort(s, date)
+  rs(s, k) = reverse(sort(s, k))
 
 defines three aliases, ``h``, ``d``, and ``rs``. ``rs(0:tip, author)`` is
 exactly equivalent to ``reverse(sort(0:tip, author))``.
 
 An infix operator ``##`` can concatenate strings and identifiers into
 one string. For example::
 
   [revsetalias]
-  issue($1) = grep(r'\bissue[ :]?' ## $1 ## r'\b|\bbug\(' ## $1 ## r'\)')
+  issue(a1) = grep(r'\bissue[ :]?' ## a1 ## r'\b|\bbug\(' ## a1 ## r'\)')
 
 ``issue(1234)`` is equivalent to ``grep(r'\bissue[ :]?1234\b|\bbug\(1234\)')``
 in this case. This matches against all of "issue 1234", "issue:1234",
 "issue1234" and "bug(1234)".
 
 All other prefix, infix and postfix operators have lower priority than
-``##``. For example, ``$1 ## $2~2`` is equivalent to ``($1 ## $2)~2``.
+``##``. For example, ``a1 ## a2~2`` is equivalent to ``(a1 ## a2)~2``.
 
 Command line equivalents for :hg:`log`::
 
   -f    ->  ::.
   -d x  ->  date(x)
   -k x  ->  keyword(x)
   -m    ->  merge()
   -u x  ->  user(x)
   -b x  ->  branch(x)
   -P x  ->  !::x
   -l x  ->  limit(expr, x)
 
 Some sample queries:
 
 - Changesets on the default branch::
 
     hg log -r "branch(default)"
 
 - Changesets on the default branch since tag 1.5 (excluding merges)::
 
     hg log -r "branch(default) and 1.5:: and not merge()"
 
 - Open branch heads::
 
     hg log -r "head() and not closed()"
 
 - Changesets between tags 1.3 and 1.5 mentioning "bug" that affect
   ``hgext/*``::
 
     hg log -r "1.3::1.5 and keyword(bug) and file('hgext/*')"
 
 - Changesets committed in May 2008, sorted by user::
 
     hg log -r "sort(date('May 2008'), user)"
 
 - Changesets mentioning "bug" or "issue" that are not in a tagged
   release::
 
     hg log -r "(keyword(bug) or keyword(issue)) and not ancestors(tag())"