Settle on a default `diff` command

[jmid]

While testing on my old macBook I realized its diff didn't support --color.
It would be nice:tm: to settle on a diff command usable across both Linux, macOS, and *BSD

Ideally, a diff command should

• support colored output - for prettier, 202* output
• support some form of --labeling - to avoid distracting users with temporary file names

In comparison, dune uses git diff over diff:
https://github.com/ocaml/dune/blob/6c573436428c2ebbb6f435b1c712777a81dc8a42/src/dune_engine/print_diff.ml#L107-L131