Add support for revsets about jj HOT 2 CLOSED

I've added initial support for revsets over the last week or two.

There's now support for looking up a single commit by git ref, such as jj diff -r origin/main (yes, there was no support for that until last week).

There is also support for looking up multiple commits. jj log gained a -r flag that can be passed a revset. Some valid revset functions are all_heads() (including those normally hidden), non_obsolete_heads() (the heads of the graph that jj log show you by default), public_heads(). The supported operators are :foo for parents, foo: for children, *:foo for ancestors and foo:* for descendants.

from jj.

There are some basic revset functions and operators and I think we can consider this done. There are of course tons of other revsets we'll want to add, but we can do that as we find a need for them. What we have now is at least enough for me to define the aliases I need for jj log etc.

The operators that exist now are:
parents: :foo
children: foo:
ancestors: ,,foo
descendants: foo,,
DAG range: foo,,bar (like hg's foo::bar)
range: foo,,,bar (like git's foo..bar)
union: foo | bar
intersection: foo & bar
difference foo - bar

Functions:
parents(<set>)
ancestors(<set>)
children(<set>)
descendants(<set>)
all_heads()
non_obsolete_heads()
public_heads()
git_refs()
merges()
description(<needle>)

I have not added any revsets for precursors or successors. I also haven't made any optimizations (e.g. rewriting ,,bar - ,,foo as foo,,,bar).

from jj.