refactor(git): improve parsing, caching, and test coverage

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

Improves Diffview’s Git parsing to better handle merge/combined diffs and tricky filenames, while extracting Git log/stat parsing into a dedicated parser module and adding functional coverage.

Changes:

• Extend diff patch parsing to recognize diff --cc / diff --combined headers and combined hunk/header formats.
• Refactor Git file-history stat parsing into vcs/adapters/git/parser.lua, and add functional tests for both parsers.
• Update Git adapter file listing/parsing to use NUL-delimited (-z) git diff output and add a small cache for Cygwin path normalization.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lua/diffview/vcs/utils.lua	Enhances diff header/hunk/index/path parsing for combined diffs.
lua/diffview/vcs/adapters/git/parser.lua	New module for structuring git log --raw/--numstat data and parsing namestat entries.
lua/diffview/vcs/adapters/git/init.lua	Uses new parser module, adds Cygwin path cache, switches tracked file parsing to -z output.
lua/diffview/tests/functional/git_parser_spec.lua	Adds functional tests for the new Git parser module.
lua/diffview/tests/functional/diff_parser_spec.lua	Adds functional tests for diff parsing, including combined diffs.

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