feat(view): add :DiffviewDiffFiles command

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

Adds support for opening a Diffview tab that compares two arbitrary files on disk (not tied to a VCS repo) via a new :DiffviewDiffFiles user command.

Changes:

• Register :DiffviewDiffFiles command + completion, and route it through diffview.diff_files().
• Introduce a NullAdapter/NullRev VCS adapter implementation and a FileDiffView specialized view for comparing two local file paths.
• Add functional tests and update docs/README/TIPS to document the new command.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
plugin/diffview.lua	Registers the new :DiffviewDiffFiles user command.
lua/diffview/init.lua	Adds diff_files() entrypoint + command-line completion handler.
lua/diffview/lib.lua	Implements lib.diffview_diff_files() that validates paths and constructs a FileDiffView.
lua/diffview/scene/views/diff/file_diff_view.lua	New view type that populates a single file entry comparing two local files.
lua/diffview/vcs/adapters/null/init.lua	Adds a “null” adapter used for non-repo diffs.
lua/diffview/vcs/adapters/null/rev.lua	Adds a corresponding NullRev implementation.
lua/diffview/tests/functional/null_adapter_spec.lua	Functional coverage for NullAdapter / NullRev.
lua/diffview/tests/functional/file_diff_view_spec.lua	Functional coverage for FileDiffView + lib.diffview_diff_files().
doc/diffview.txt	Documents :DiffviewDiffFiles.
TIPS.md	Adds a usage tip and shell function example.
README.md	Mentions the new command in usage and lazy-loading examples.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.