[UI] Re-prime the stack preview when the status behind it changes

[KCaverly]

Refreshing with the preview pane open left it on "loading diff…" forever.
applyStackStatus drops the per-commit diff cache — correctly, since a
restack or amend can move SHAs out from under it — but nothing re-issued
the fetch, so the pane went on looking up a key that would never be filled.
Only nudging the cursor onto another commit brought it back. Pressing r,
the most natural thing to do while reading a diff, was enough to hit it.

The four status-bearing results now re-kick the cursored commit's fetch
through ensureSplitDiff, which no-ops unless the preview is actually on
screen. Doing it at the message handler rather than inside the apply
helpers keeps them free of command plumbing and covers submit, restack and
merge — restack lands a status whose SHAs are all new, so it misses the
cache the same way a refresh does.

A real restack also drops the cache outright now, so a resurrected SHA
cannot serve a pre-restack patch. The dry run leaves it alone; it only
renders a plan.

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

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Stack (bottom → top):

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