fix(diff): check modified count in identical-files guard

[gghez]

Summary

rtk diff incorrectly reports "Files are identical" when lines differ by only a few characters. Two root causes:

1. The identical-files guard at line 24 only checked added == 0 && removed == 0, ignoring the modified counter. Lines classified as modifications (similarity > 0.5) incremented modified but not added/removed, so the guard passed and the diff silently swallowed actual differences.

2. The similarity() function used Jaccard similarity on unique character sets, which over-estimated similarity for lines sharing common characters regardless of position. Replaced it with a positional prefix+suffix ratio that better reflects actual line differences.

Changes

• Added && diff.modified == 0 to the identical-files guard in run()
• Replaced similarity() implementation: Jaccard set-based -> positional prefix+suffix metric
• Added 4 regression tests covering all reproduction cases from the issue
• Updated existing test_similarity_partial_overlap for the new metric

Test plan

• All 983 existing tests pass (cargo test --all)
• 4 new regression tests cover exact reproduction cases from diff reports "Files are identical" when lines differ by only a few characters #781
• cargo check compiles without warnings

Fixes #781

[CLAassistant]

All committers have signed the CLA.

[pszymkowiak]

[w] wshm · Automated triage by AI

📊 Automated PR Analysis

🐛 Type	bug-fix
🟡 Risk	medium

Summary

Fixes a bug where rtk diff falsely reported files as identical when lines differed by only a few characters. The identical-files guard now checks the modified counter in addition to added and removed, and the similarity function was replaced with a positional prefix+suffix metric to avoid over-estimating similarity.

Review Checklist

• Tests present
• Breaking change
• Docs updated

Linked issues: #781

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Analyzed automatically by wshm · This is an automated analysis, not a human review.