RFC: revset relation operator

[Matt Mackall]

It would be nice if we had a convenience syntax for finding things
related to a given changeset. Currently, we have a couple of these for
parents, ancestors, and descendants.

 foo^  # first parent
 foo^2 # second parent
 foo^^ # first parent of first parent
 foo~2 # same
 ::foo # ancestors, inclusive
 foo:: # descendants, inclusize

But we have nothing directly equivalent to parents(), nor any shorthand
for children(), successors(), origin(), destination(), or any other type
of relation we may add. We'd also like a way to specify next, previous,
all, all exclusive, last, etc.

So my proposal[1] is to add a bracketed postfix (one of {}, [], or <>,
but perhaps {} is the least bad) that looks like this:

 foo{1}     # children(foo)
 foo{2}     # children(children(foo))
 foo{}      # defaults to 1, so children(foo)
 foo{0}     # synonym for foo
 foo{-1}    # parents(foo) (both of 'em)
 foo{-}     # same
 foo{*}     # foo:: - foo (exclusive)
 foo{**}    # foo:: (inclusive)
 foo{-*}    # ::foo - foo
 foo{$}     # heads(foo::) aka "the last children"
 foo{-4}::foo # last five changesets in a branch

This makes a lot of our most common expressions shorter. To talk about
other types of relations, we use a suffix character:

 foo{1g}    # immediate grafts of foo
 foo{-g}    # origin for grafts
 foo{o}     # successor(foo)
 foo{$o}    # final successor of foo

Note a consistent direction in the sign used regardless of relation:
past is negative, future is positive. Operations can be chained:

 foo{-}{}   # foo and its siblings
 foo{$o}{}  # first children of final successor

We might also add a long-hand method:

 related(foo, "$", "o")

This lets you build revset aliases of various sorts more conveniently
and we can transform the short expressions into this form internally.

All the bracket choices have problems:

 {}: used by git very differently, might need shell quoting
 []: used differently by languages, might need shell quoting
 <>: might need shell quoting, can have ugly side-effects

The little sublanguage in the brackets might present some new wrinkles
for our parser, which doesn't like most of the characters there anyway.

[1] largely inspired by one from Durham and a discussion at the last
sprint
-- 
Mathematics is the supreme nostalgia of our time.