help: apply the section headings from revsets to filesets

This has the nice property of visually breaking up the wall of text.  It also
allows specific smaller sections to be called out.  For example,
`hg help filesets.predicates` now prints just the predicate section.  At the
moment, the revset headings are a superset of the fileset headings, so there is
consistency in how example, predicate and operator help is called out.

The reference to `hg help patterns` was moved to the overview section, so that
it isn't stuck in the examples section.

mercurial/help/filesets.txt

@@ -15,11 +15,19 b' Special characters can be used in quoted'
 e.g., ``\n`` is interpreted as a newline. To prevent them from being
 interpreted, strings can be prefixed with ``r``, e.g. ``r'...'``.
 
+See also :hg:`help patterns`.
+
+Prefix
+======
+
 There is a single prefix operator:
 
 ``not x``
   Files not in x. Short form is ``! x``.
 
+Infix
+=====
+
 These are the supported infix operators:
 
 ``x and y``
@@ -32,10 +40,16 b' These are the supported infix operators:'
 ``x - y``
   Files in x but not in y.
 
+Predicates
+==========
+
 The following predicates are supported:
 
 .. predicatesmarker
 
+Examples
+========
+
 Some sample queries:
 
 - Show status of files that appear to be binary in the working directory::
@@ -61,5 +75,3 b' Some sample queries:'
 - Remove files listed in foo.lst that contain the letter a or b::
 
     hg remove "set: 'listfile:foo.lst' and (**a* or **b*)"
-
-See also :hg:`help patterns`.