feat: add --base flag for branch comparison diffs

[star-gusto]

What Is This Change

Adds a -b, --base <ref> CLI option that shows the diff between HEAD and a base ref (e.g. main, origin/main, a commit SHA) using git diff <base>...HEAD, rendered in Diffly's GitHub-like browser UI.

# compare current branch against main
diffly --base main

# compare against a specific commit
diffly --base abc1234

# combine with port flag
diffly -p 9393 --base origin/main

Without --base, behavior is unchanged -- Diffly shows working tree changes (staged, unstaged, untracked) as before.

Why Is This Change Being Made

Currently Diffly only shows uncommitted working tree changes. Once changes are committed, there's nothing to display. This makes Diffly less useful for the common workflow of reviewing a feature branch's full diff before opening a PR.

The --base flag fills that gap by letting users view committed branch diffs in Diffly's split-view UI -- the same way they'd see them on GitHub, but locally and instantly.

Tests

Tested manually against a real feature branch, affecting 9 files, in a large Rails monorepo:

• diffly --base main correctly shows 9 changed files with full split-view diffs
• /api/diff response includes base field identifying the ref
• Without --base, existing behavior (working tree changes) is preserved
• Invalid base ref produces a clear simple-git error message
• Port flag (-p) works correctly alongside --base

References

• Commander.js .option() docs
• git diff <base>...HEAD (three-dot) semantics

[star-gusto]

@Shugar If you could take a look at this PR when you can, I'd appreciate it!