investigate `cargo insta` and copy it from jj CI

[apoelstra]

The jujutsu project uses something called "cargo insta" to snapshot its CLI output and test against it. We should copy whatever they're doing.

cf https://github.com/jj-vcs/jj/actions/runs/15764112613/job/44437016478?pr=6790 (will probably expire)

[apoelstra]

Ok, there is not much value to cargo-insta for such a simple tool. It will automatically generate snapshots etc., but our whole API surface is small enough that setting this up would probably take longer than just manually snapshotting everything. (Also jj-vcs, which uses cargo-insta, seems to have inline "manual" snapshots in its unit tests anyway, which make me wonder what the value proposition of cargo-insta is.)

jj also uses assert_cmd, but similarly this seems to have limited value. It provides some facilities for obtaining the current binary name, but this is available in environment variables https://docs.rs/assert_cmd/latest/src/assert_cmd/macros.rs.html#46-50 so it's not clear what the crate gets us over writing a 1-line function to just get that variable.

But ok, I will write a bunch of unit tests in this model.

[apoelstra]

Fixed by #6.