fix(file): load existing unloaded buffer in _create_local_buffer

[dlyongemallo]

Makes rapidly switching files (by pressing <tab>/<s-tab>) much more stable.

[Copilot]

Pull request overview

This PR improves stability when rapidly switching between files by ensuring Diffview doesn’t reuse an existing unloaded file buffer (created by other plugins via bufadd()-style behavior) without first loading its contents from disk.

Changes:

• In vcs.File:_create_local_buffer(), force-load an existing matching buffer via bufload() when it is currently unloaded.
• Add a functional regression test covering the “pre-existing unloaded buffer” scenario for RevType.LOCAL.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lua/diffview/vcs/file.lua	Ensures reused LOCAL buffers are loaded from disk before diff rendering, preventing empty-placeholder reuse.
lua/diffview/tests/functional/file_spec.lua	Adds a regression test that reproduces an unloaded named buffer and verifies Diffview loads its content.

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