fix(udiff): normalize paths for every file header

[KXHXK]

Summary

• normalize standard Git a/ and b/ prefixes for every file header pair in a fenced unified diff
• reuse the same path handling for the initial and subsequent files
• add regression coverage for a two-file diff

Fixes #5560

Test plan

• python -m pytest tests/basic/test_udiff.py -q
• python -m flake8 aider/coders/udiff_coder.py tests/basic/test_udiff.py
• git diff --check

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