compiletest: Not all `diff` installations have `--color`

[camelid]

There is some rustdoc-related code in compiletest that uses the diff command-line tool; however, it passes the --color flag to diff, which diff does not accept on my system (macOS). We should probably use git diff --no-index instead of diff since it will have colors and will automatically use a pager. We may also want to pass the --minimal flag to diff/git diff; the manual page says:

--minimal
Spend extra time to make sure the smallest possible diff is produced.

See also #79991 (comment).

cc @jyn514

[jyn514]

@camelid was this fixed by #82469 ?

[notriddle]

rust/src/tools/compiletest$ rg -- '--color'
src/main.rs
191:        Some(x) => panic!("argument for --color must be auto, always, or never, but found `{}`", x),

Yeah, it looks like it's fixed.