[UI] Cover the stack diff preview pane with tests

[KCaverly]

Exercises the split preview added in the previous commit, plus the two git
helpers behind it.

internal/tui: d toggles the pane on and back off with the overlay intact;
tab flips focus and swaps the footer; j (list focus) and n/p (diff focus)
walk commits and fetch only on a cache miss, with n/p staying in the diff
pane; esc steps out of the split before closing; an arriving patch populates
the cache and renders, while one keyed to another worktree is dropped; the
scroll and file-jump keys are inert without a cached patch rather than
panicking; a refresh drops cached patches. Regression guards keep enter
inert on a row and leave v routed to the whole-branch diff, and a
fixed-geometry assertion pins the two-column layout.

internal/repo: DiffCommit/NumstatCommit over a throwaway repo, covering both
the ordinary sha^..sha case and the root-commit fallback. These live in
integration_test.go with the other git-backed tests, keeping repo_test.go
free of a git dependency.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

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Stack (bottom → top):

1. [UI] Add a per-commit diff preview pane to the stack screen #36
2. [UI] Cover the stack diff preview pane with tests #37 ← this PR
3. [Git] Stop inlining binary literals in the per-commit diff #38
4. [UI] Re-prime the stack preview when the status behind it changes #39
5. [Git] Stop returning quoted, octal-escaped untracked paths #40
6. [Git] Add DiffUntracked to render untracked file contents #41
7. [UI] Show uncommitted work as a row in the stack #42
8. [UI] Name one way out of the stack preview #43
9. [UI] Give d the preview and esc the way out #44
10. [UI] Drop the branch-diff jump from the stack screen #45
11. [UI] Stop the stack preview overrunning a narrow terminal #46
12. [Git] Pin the diff flags the body parser depends on #47
13. [UI] Teach both diff viewers the same file-jump key #48
14. [Diff] Thread the [diff] config through the viewer and zoom with z #49
15. [Git] Carry diff args and excludes through every diff call #50
16. [Diff] Add the external diff pager package #51
17. [Config] Accept a [diff] section with an optional pager #52
18. [Docs] Document the diff viewer settings #53