fix for rev-parse error when rebasing over deleted branches

[skarim]

Fix rev-parse error when rebasing over deleted branches

When a merged branch's remote ref was deleted (e.g. "Automatically delete head branches"), gh stack rebase would fail with a rev-parse error because it tried to resolve refs for all branches, including ones that no longer exist.

Fix: Filter out merged branches that don't exist locally before calling RevParseMulti to snapshot original refs. These branches are already skipped during the actual rebase cascade.

[Copilot]

Pull request overview

This PR addresses a failure during gh stack rebase when the stack contains merged branches whose local refs no longer exist (e.g., branch auto-deleted after merge), by avoiding git rev-parse on refs that can’t be resolved.

Changes:

• Skip merged branches that don’t exist locally when building the ref list for RevParseMap, preventing rev-parse errors.
• Update existing rebase tests to account for the new BranchExists behavior.
• Add a new regression test covering the “merged + deleted branch” scenario.

Show a summary per file

File	Description
cmd/rebase.go	Excludes merged non-existent branches from the batch RevParseMap resolution to prevent rev-parse failures.
cmd/rebase_test.go	Adjusts mocks for the new branch-existence check and adds a regression test for deleted merged branches.

Copilot's findings

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