fix for inflated diff counts when base branch has been updated since stack init

[skarim]

Fix inflated diff counts in gh stack view TUI

When the local base branch (e.g. main) had advanced past a branch's fork point, gh stack view showed inflated file counts and diff stats because the two-dot diff compared full trees instead of diffing from the fork point.

Fix: Always use git merge-base to find the fork point for diff computation, not just for merged branches.

[Copilot]

Pull request overview

Fixes incorrect/inflated diff file counts in the stack view when the base branch has advanced since the stack was initialized by anchoring diffs to the merge-base (fork point) instead of the base branch tip.

Changes:

• Update LoadBranchNodes to compute diff/commit ranges using git merge-base(base, branch) as the diff base.
• Add unit tests covering diverged-base and linear-history scenarios to ensure merge-base is used.

Show a summary per file

File	Description
internal/tui/stackview/data.go	Switches diff anchor to merge-base to avoid including unrelated base-branch changes in file stats.
internal/tui/stackview/data_test.go	Adds regression tests validating merge-base behavior for diverged and linear cases.

Copilot's findings

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• Files reviewed: 2/2 changed files
• Comments generated: 1