revset: add experimental relation and subscript operators The proposed syntax [1] was originally 'set{n rel}', but it seemed slightly confusing if template is involved. On the other hand, we want to keep 'set[n]' for future extension. So this patch introduces 'set#rel[n]' ternary operator. I chose '#' just because it looks like applying an attribute. This also adds stubs for 'set[n]' and 'set#rel' operators since these syntax elements are fundamental for constructing 'set#rel[n]'. [1]: https://www.mercurial-scm.org/wiki/RevsetOperatorPlan#ideas_from_mpm

--- a/tests/test-annotate.t	Sat Jul 08 12:49:46 2017 +0900
+++ b/tests/test-annotate.t	Sat Jul 08 13:07:59 2017 +0900
@@ -812,7 +812,7 @@
   abort: line range exceeds file size
   [255]
   $ hg log -r 'followlines(baz, 2:4, startrev=20, descend=[1])'
-  hg: parse error at 43: syntax error in revset 'followlines(baz, 2:4, startrev=20, descend=[1])'
+  hg: parse error at 43: not a prefix: [
   [255]
   $ hg log -r 'followlines(baz, 2:4, startrev=20, descend=a)'
   hg: parse error: descend argument must be a boolean