fix(restack): handle GC'd fork points gracefully with a warning

When a stored fork point commit has been garbage collected (e.g. after
a history rewrite followed by aggressive `git gc`), the previous code
silently fell through to a plain rebase. Now it emits a warning so the
user knows why `--onto` wasn't used, then falls back to a simple
rebase as before.

Author: boneskull

cmd/cascade.go

@@ -164,30 +164,46 @@ func doCascadeWithState(g *git.Git, cfg *config.Config, branches []*tree.Node, d
 			continue
 		}
 
-		// Check if we should use --onto rebase
-		// This is needed when parent has been rebased/amended since child was created
+		// Check if we should use --onto rebase.
+		// This is needed when the parent has been rebased/amended since the child was created.
 		storedForkPoint, fpErr := cfg.GetForkPoint(b.Name)
 		useOnto := false
 
-		if fpErr == nil && g.CommitExists(storedForkPoint) {
-			currentMergeBase, mbErr := g.GetMergeBase(b.Name, parent)
-			if mbErr == nil && currentMergeBase != storedForkPoint {
-				// Fork point differs from merge-base. Determine why:
-				//
-				// If the stored fork point is an ancestor of the merge-base,
-				// it's just stale (e.g. branch was rebased outside gh-stack,
-				// or fork point wasn't updated after a conflict resolution).
-				// A simple rebase using the merge-base is correct; refresh the
-				// fork point so it stays current.
-				//
-				// If the stored fork point is NOT an ancestor of the merge-base,
-				// the parent's history was rewritten (squash merge, force push).
-				// We need --onto with the stored fork point to identify the
-				// correct commit range.
-				if g.IsAncestor(storedForkPoint, currentMergeBase) {
-					_ = cfg.SetForkPoint(b.Name, currentMergeBase) //nolint:errcheck // best effort
-				} else {
-					useOnto = true
+		if fpErr == nil {
+			if !g.CommitExists(storedForkPoint) {
+				// The stored fork point no longer exists — it may have been
+				// garbage collected after a history rewrite or a sufficiently
+				// aggressive `git gc`. Without it we cannot use --onto, so we
+				// fall back to a plain rebase against the parent tip. If the
+				// parent's history was genuinely rewritten this fallback may
+				// produce spurious conflicts or silently re-apply commits that
+				// were already in the parent; the user should resolve conflicts
+				// as normal or re-run `gh stack restack` after history settles.
+				fmt.Printf("  %s\n", s.Muted(fmt.Sprintf(
+					"warning: stored fork point %s is no longer available (garbage collected?); falling back to simple rebase",
+					git.AbbrevSHA(storedForkPoint),
+				)))
+			} else {
+				// Fork point is reachable — determine whether --onto is appropriate.
+				currentMergeBase, mbErr := g.GetMergeBase(b.Name, parent)
+				if mbErr == nil && currentMergeBase != storedForkPoint {
+					// Fork point differs from merge-base. Determine why:
+					//
+					// If the stored fork point is an ancestor of the merge-base,
+					// it's just stale (e.g. branch was rebased outside gh-stack,
+					// or fork point wasn't updated after a conflict resolution).
+					// A simple rebase using the merge-base is correct; refresh the
+					// fork point so it stays current.
+					//
+					// If the stored fork point is NOT an ancestor of the merge-base,
+					// the parent's history was rewritten (squash merge, force push).
+					// We need --onto with the stored fork point to identify the
+					// correct commit range.
+					if g.IsAncestor(storedForkPoint, currentMergeBase) {
+						_ = cfg.SetForkPoint(b.Name, currentMergeBase) //nolint:errcheck // best effort
+					} else {
+						useOnto = true
+					}
 				}
 			}
 		}