lib: allow exactly() to take interval ranges as an argument

[thoughtpolice]

It turns out that the use case I had for exactly() was slightly more subtle, and I need range support. In particular, usage of exactly() in expressions used by revsets.log-graph-prioritize can and will cause jj log to fail if the assertion fails; in my case, a particular expression I use should be equal to zero-or-one revisions.

This (ab)uses string literal syntax to parse out a range that is like a subset of Rust's range syntax; in particular unbounded endpoints like x.. or ..=y are not supported. Ideally the grammar would be expanded to handle this case more smoothly.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes

[thoughtpolice]

Note that I didn't take a stab at extending the parser yet to support range literals; the string-literal syntax is gross but I wanted to get the patch up first. But I can do that if we think we should (I suspect @yuja has thoughts :)

[yuja]

My concern of reusing x..y syntax is that x..y in revset is kind of a half-open range of excluded/included pair, whereas integer range is included/excluded pair. I don't feel strongly whether it would be a good idea to introduce integer range of different semantics, though.

If we aren't sure, maybe we can add exactly(x, min, [max=min]) or exactly(x, min, [count=1]).

Another option is to add infallible version of exactly(x, n) which evaluates to none(), and use exactly(x, 0) | exactly(x, 1).