feat(config): add options visual appearance options

[dlyongemallo]

Relates to #70.

[Copilot]

Pull request overview

Adds new configuration knobs to tune Diffview’s UI styling/visual noise, aligning the fork’s appearance closer to the original sindrets/diffview.nvim expectations (Issue #70).

Changes:

• Add file_panel.tree_options.folder_count_style and file_panel.tree_options.folder_trailing_slash to control collapsed-folder counts and folder name formatting.
• Add file_history_panel.subject_highlight to control ref-aware vs plain commit subject highlighting.
• Document the new options in doc/diffview.txt and doc/diffview_defaults.txt.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
lua/diffview/scene/views/file_history/render.lua	Gates commit subject highlight behavior behind new subject_highlight option.
lua/diffview/scene/views/diff/render.lua	Adds new tree rendering options for folder trailing slash and count style.
lua/diffview/config.lua	Introduces new default config values for the added UI options.
doc/diffview.txt	Documents the new configuration options.
doc/diffview_defaults.txt	Updates the default-config documentation output with the new options.

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[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated no new comments.

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[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated no new comments.

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