FR: Introduce a filesets expression language

[thoughtpolice]

Many commands that take paths naturally take multiple paths, and sometimes expressing the proper paths you want is awkward because there is no way to do so beyond giving the literal path itself. You may even do some inline shell scripting to figure out the set of files for some commands; the horror...

In short, commands on path can also (mostly, in theory) operate on sets of paths, the same way many functions that take revisions also often work on sets of revisions. And we have an expression language we use to compose those revision sets in a natural way: revsets, which we all know and love. Instead, we want a composable expression language for paths instead.

Mercurial calls this feature filesets.

As an example, #2286 describes a problem where if you have 100 changed files, and want to jj split the files into a pair of commits with 99 commits in the first and 1 in the second, it's really awkward. However, with a fileset we could imagine a command like jj split -p 'not(./foo.txt)' in order to achieve the desired result instantly.

Similarly, #2915 described a request to exclude files from a diff. This is as easy as -p not(Cargo.lock) using the same imaginary syntax. (Perhaps even a config such as ui.diff.ignored-filesets = "not(Cargo.lock)" could be created that is read by jj diff automatically, though that's not relevant to this particular ticket.)

Many commands that then take a path or series of path arguments can then be migrated to use -p style flags that take fileset expressions.

Yuya has noted some of the syntax and issues with Mercurial's approach here: #2915 (comment)

Waleed also called these "pathsets" in #2915. We already reuse the word "revset" from Mercurial, so I think reusing "filesets" is fine, though I kind of like "pathset" too...