feat: enable merge-base comparison between working tree and specific ref

[KEY60228]

Summary

Enable three-dot diff (merge-base comparison) when comparing working tree against a specific ref

Changes

• Add get_merge_base() function to find the common ancestor between HEAD and a reference
• Update get_diff_files() to use merge base when comparing working tree against a ref
• Configure alt-diffview to use --merge-base option for accurate comparison
• Add warning for diffview.nvim users about two-dot diff limitation
• Fix typo in review start message (double dots to triple dots)

Motivation

When reviewing changes against a base branch (e.g., main), users typically want to see only the changes introduced in their current branch, not all differences between branches. This PR implements three-dot diff behavior (base...HEAD) instead of two-dot diff (base..HEAD) when comparing the working tree against a reference.

Technical Details

• For alt-diffview users: Utilizes the --merge-base option for accurate three-dot comparison
• For diffview.nvim users: Shows a warning since it doesn't support merge-base comparison
• The merge base calculation only applies when comparing working tree against a ref (not for ref-to-ref comparisons)

Testing

• Tested with alt-diffview: correctly shows only branch changes
• Tested with diffview.nvim: warning displayed, falls back to two-dot diff
• Verified merge base calculation works correctly
• Existing ref-to-ref comparisons remain unchanged