adopt diff: byte-accurate identity, stop false 'identical' verdicts (upstream #3268) #100

Upstream PR: rtk-ai#3268 by @iliaal.

Bug: rtk diff decides identity from normalized str::lines(), so CRLF/final-newline differences report [ok] Files are identical with exit 0; alignment is a positional loop instead of LCS. Rank (a)+(b): false verdict plus wrong exit code, and plain diff is auto-rewritten into this path.

Verified on develop (2026-08-17 sweep): src/cmds/git/diff_cmd.rs:38-43 line-based identity, :109-111 positional loop.

Related upstream duplicates: rtk-ai#3471 (byte-accurate compare) and rtk-ai#3519 (byte-different check) patch the same site — adopt rtk-ai#3268 as the most complete, reference the others.