revset: add experimental relation and subscript operators...
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
