feat(config): add view.foldlevel and default to 0

[dlyongemallo]

See #70 (comment).

[Copilot]

Pull request overview

Adds a new user-facing configuration option to control the initial fold level in diff windows, aligning folding behavior with upstream defaults while still allowing users to keep folds open when needed.

Changes:

• Introduces view.foldlevel (default 0) with validation in config.setup().
• Applies the configured foldlevel when opening diff buffers/windows.
• Updates docs/tips and adds config-focused functional tests for the new option.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lua/diffview/vcs/file.lua	Changes default diff-window foldlevel winopt baseline to 0 and notes it’s resolved at open time.
lua/diffview/scene/window.lua	Applies conf.view.foldlevel to the file window options prior to saving/restoring winopts.
lua/diffview/config.lua	Adds default view.foldlevel and validates it as a non-negative integer.
lua/diffview/tests/functional/config_features_spec.lua	Adds functional tests covering default/override/invalid values for view.foldlevel.
doc/diffview_defaults.txt	Documents the new default config key.
doc/diffview.txt	Adds help documentation for view.foldlevel.
TIPS.md	Updates vim-markdown workaround guidance to use view.foldlevel.

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[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

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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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