fix push for branches with stale tracking refs

[skarim]

• Fixes gh stack sync can fail force-with-lease push after fetching stack branches #118

gh stack sync can rebase a stack successfully, then fail the final force push with stale info when a branch lacks a local tracking ref (refs/remotes/<remote>/<branch>). This is most visible in checkouts where local tracking refs have been pruned or never existed.

Root cause is two layers:

1. FetchBranches pre-filters branches by existing tracking ref, so a branch with no tracking ref is never fetched and never gains one.
2. Push uses a bare --force-with-lease flag, which has no lease basis for a branch without a tracking ref — git rejects the push.

The fix addresses both layers while preserving --force-with-lease safety (never overwrite work we have not seen).

Behavior matrix (after fix)

Branch state	Tracking ref after fetch	Lease emitted	Outcome
Exists remotely, in sync	yes, current	explicit SHA	force push succeeds
Exists remotely, advanced by someone else	yes, current	explicit SHA	rejected (correct, protects their work)
Exists remotely, no local tracking ref before	yes (created by fetch fix)	explicit SHA	force push succeeds (the bug fix)
Absent remotely (new branch)	no	empty expect	created, or rejected if raced (safe)

Changes

internal/git/gitops.go:

• FetchBranches: Remove pre-filter that skipped branches without existing tracking refs. Build explicit refspecs (+refs/heads/<branch>:refs/remotes/<remote>/<branch>) for every requested branch, creating or updating tracking refs regardless of prior state. The + prefix allows non-fast-forward tracking-ref updates. Keep the existing fast-path (single fetch for all branches) and per-branch fallback (one missing remote branch does not block the rest). Per-branch failures are tolerated — a branch absent on the remote simply gets no tracking ref, which is correct.
• Push: When force=true, resolve each branch's tracking ref via rev-parse --verify --quiet refs/remotes/<remote>/<branch>. If found, emit --force-with-lease=refs/heads/<branch>:<sha> (explicit SHA lease). If missing (branch genuinely absent on remote), emit --force-with-lease=refs/heads/<branch>: (empty expected value = "must not exist"). Use explicit destination refspecs (<branch>:refs/heads/<branch>) to remove dependence on push.default and upstream configuration. The force=false path is unchanged.

internal/git/gitops_test.go:

• Add 6 integration test scenarios using real bare git remotes (not mocks):
  1. Branch exists remotely with current tracking ref — push succeeds, remote updated
  2. Branch exists remotely, tracking ref deleted locally — push succeeds after fetch fix (regression test for gh stack sync can fail force-with-lease push after fetching stack branches #118)
  3. Branch advanced on remote by another client after fetch — push rejected (lease protects their work)
  4. New branch absent on remote — created via empty-expect lease
  5. New branch race condition (another client created it first) — push rejected
  6. Mixed stack: one branch with tracking ref + one without — both succeed after fetch fix

Other callers

• cmd/push.go calls FetchBranches then Push with force=true — benefits automatically
• cmd/submit.go calls Push with force=true per-branch — benefits automatically
• cmd/link.go calls Push with force=false — unaffected (non-force path unchanged)

---

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[Copilot]

Pull request overview

This PR fixes issue #118 where gh stack sync could fail during force push when branches lack local tracking refs (refs/remotes/<remote>/<branch>). The fix addresses two root causes: FetchBranches skipping branches without existing tracking refs, and Push using bare --force-with-lease which has no lease basis without a tracking ref.

Changes:

• FetchBranches now builds explicit refspecs for all requested branches, creating/updating tracking refs regardless of prior state
• Push with force=true now resolves each branch's tracking ref SHA and uses explicit --force-with-lease=refs/heads/<branch>:<sha> arguments with explicit destination refspecs
• Added 6 integration tests covering all scenarios (existing branch, missing tracking ref, remote advanced, new branch, race condition, mixed stack)

Show a summary per file

File	Description
internal/git/gitops.go	Rewrites FetchBranches to remove tracking-ref pre-filter and use explicit refspecs; rewrites Push force path to use per-branch explicit lease SHAs and refspecs
internal/git/gitops_test.go	Adds integration tests using real bare git repos covering all combinations of tracking ref state and remote branch state

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• Files reviewed: 2/2 changed files
• Comments generated: 0

[georgebrock]

The new approach makes sense to me 👍