fix(view): refresh tracked HEAD when right is STAGE

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

This PR fixes DiffView’s rev refresh behavior so that when the left side is tracking HEAD, it will refresh to the new HEAD even if the right side is STAGE (cached/staged view), preventing diffs from being computed against a stale base commit.

Changes:

• Update DiffView.update_files() to refresh the tracked HEAD rev whenever self.left.track_head is set (no longer gated on right being LOCAL).
• Add a functional regression test that reproduces the stale-HEAD issue for cached/staged views and asserts left updates after a commit.

Reviewed changes

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

File	Description
lua/diffview/scene/views/diff/diff_view.lua	Refresh tracked HEAD regardless of right-side rev type, fixing staged/cached views keeping a stale left base.
lua/diffview/tests/functional/diff_view_spec.lua	Adds regression test ensuring left updates when HEAD moves while a right=STAGE view remains open.

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