diff paths are confusing when using `jj diff`

[CobaltCause]

This issue is exactly the same as #620 except for jj rather than git, where leading directory components are not shown by difftastic. A case where this happens for me is when jj invokes difft like this:

INFO run_command:cmd_diff: jj_cli::merge_tools::external: Invoking the external diff generator: cmd=cd "/tmp/jj-diff-McWduD" && "difft" "--color=always" "left/crates/derail/src/report/buffer.rs" "right/crates/derail-report/src/buffer.rs"

(This issue can be worked around by not setting merge-tools.difft.diff-invocation-mode = "file-by-file" in jj's configuration, but this has the downside of handling renamed files by showing the original file as deleted and the replacement as though it were a completely new and unrelated file.)

Version info

$ difft --version
Difftastic 0.64.0

Toolchain: 1.87.0
System:    linux x86_64

On NixOS unstable

[Wilfred]

Just to be explicit here: what path is it printing, and what would be the ideal output?

[CobaltCause]

Ah, sorry. Difftastic chooses to show just buffer.rs as the header for that file's diff, and doesn't mention it was renamed from anything. I think ideally it would show crates/derail-report/src/buffer.rs as the header and mention that it was renamed from crates/derail/src/report/buffer.rs.

(Also, thinking out loud, I noticed that difftastic has some pretty specific code for handling this kind of case with git, and I wonder if there might be a more general way to fix this kind of thing by e.g. exposing some extra command line options.)