feat(config): support linematch in diffopt override

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

Adds support for overriding Neovim’s diffopt linematch setting while a Diffview tab is open, aligning config/docs/tests with the new capability.

Changes:

• Extend per-view diffopt override logic to handle linematch:N (replace any existing linematch:* entry).
• Add functional tests covering apply/replace/no-op behavior for linematch overrides.
• Document the new diffopt.linematch config key in both Lua defaults and :h diffview-config-diffopt.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
lua/diffview/scene/view.lua	Applies diffopt.linematch overrides by removing existing linematch:* and appending linematch:<N> while the view is active.
lua/diffview/tests/functional/view_spec.lua	Adds functional coverage for configuring/replacing/restoring linematch in diffopt overrides.
lua/diffview/config.lua	Updates defaults comment to include linematch as a supported diffopt override key.
doc/diffview.txt	Documents diffopt.linematch, its purpose, and updates the example config.

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