fix for rev-parse error during sync with deleted branches

[skarim]

Fix rev-parse error during sync with deleted branches

Same root cause as the rebase fix, but in the gh stack sync code path. The branch-ref snapshot before rebase included deleted branches, causing a rev-parse failure.

Fix: Apply the same guard — skip merged branches that don't exist locally when building the ref snapshot.

[Copilot]

Pull request overview

Updates gh-stack’s sync/rebase behavior and documentation around handling merged PRs (previously described as “squash-merged”), including a new test case for merged branches that no longer exist locally (e.g., deleted after merge).

Changes:

• Adjusts sync’s pre-rebase “original refs” capture to skip merged branches that don’t exist locally.
• Renames “squash-merged” terminology to “merged” across CLI docs, README, skill docs, messages, and tests.
• Adds/updates tests to cover merged-branch --onto behavior and a “merged branch deleted from remote” scenario.

Show a summary per file

File	Description
skills/gh-stack/SKILL.md	Updates user-facing documentation wording from squash-merge to merged PR handling.
internal/git/git.go	Updates RebaseOnto doc comment terminology to “merged”.
docs/src/content/docs/reference/cli.md	Updates CLI reference text to describe merged PR detection for --onto.
cmd/sync_test.go	Renames tests and adds coverage for merged-branch scenarios, including missing local branch.
cmd/sync.go	Skips non-existent merged branches when collecting SHAs for conflict-restore bookkeeping.
cmd/rebase_test.go	Renames tests/comments to match “merged” terminology.
cmd/rebase.go	Updates comments/messages to match “merged” terminology.
README.md	Updates README description of merged PR handling during rebase.

Copilot's findings

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cmd/rebase.go:209

• The merged-branch skip path sets ontoOldBase = originalRefs[br.Branch]. If a merged branch was deleted locally and therefore excluded from the RevParseMap inputs, this lookup will be empty and subsequent RebaseOnto(newBase, ontoOldBase, ...) will run with an invalid oldBase. Consider using the persisted SHA from stack metadata (e.g., br.Head) when the ref is missing, and fail fast with a clear message if you can't determine an oldBase SHA.

		// Skip branches whose PRs have already been merged.
		// Record state so subsequent branches can use --onto rebase.
		if br.IsMerged() {
			ontoOldBase = originalRefs[br.Branch]
			needsOnto = true
			cfg.Successf("Skipping %s (PR %s merged)", br.Branch, cfg.PRLink(br.PullRequest.Number, br.PullRequest.URL))

• Files reviewed: 8/8 changed files
• Comments generated: 3