Add regression for duplicate-patch rebase stop

Co-authored-by: Andrew <andrewxhill@gmail.com>

Author: andrewxhill-ai

internal/git/rebase_test.go

@@ -0,0 +1,41 @@
+package git
+
+import (
+	"errors"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestRebaseCurrentBranchStopsWhenPatchAlreadyExistsUpstream(t *testing.T) {
+	repoRoot := t.TempDir()
+	runGitCommand(t, repoRoot, "git", "init", "-b", "main")
+	runGitCommand(t, repoRoot, "git", "config", "user.name", "Test User")
+	runGitCommand(t, repoRoot, "git", "config", "user.email", "test@example.com")
+	runGitCommand(t, repoRoot, "git", "config", "commit.gpgsign", "false")
+	runGitCommand(t, repoRoot, "git", "config", "tag.gpgsign", "false")
+	runGitCommand(t, repoRoot, "git", "config", "core.hooksPath", ".git/hooks")
+
+	writeFile(t, filepath.Join(repoRoot, "README.md"), "# test\n")
+	runGitCommand(t, repoRoot, "git", "add", "README.md")
+	runGitCommand(t, repoRoot, "git", "commit", "-m", "initial")
+
+	runGitCommand(t, repoRoot, "git", "checkout", "-b", "feature/duplicate-patch")
+	writeFile(t, filepath.Join(repoRoot, "README.md"), "# test\nqueued change\n")
+	runGitCommand(t, repoRoot, "git", "commit", "-am", "feature patch")
+
+	runGitCommand(t, repoRoot, "git", "checkout", "main")
+	writeFile(t, filepath.Join(repoRoot, "README.md"), "# test\nqueued change\n")
+	runGitCommand(t, repoRoot, "git", "commit", "-am", "upstream patch")
+
+	runGitCommand(t, repoRoot, "git", "checkout", "feature/duplicate-patch")
+	err := NewEngine(repoRoot).RebaseCurrentBranch(repoRoot, "main")
+	if !errors.Is(err, ErrRebaseEmpty) {
+		t.Fatalf("expected ErrRebaseEmpty for duplicate patch rebase, got %v", err)
+	}
+
+	status := runGitCommand(t, repoRoot, "git", "status")
+	if !strings.Contains(status, "rebase in progress") {
+		t.Fatalf("expected repository to remain in rebase state, got %q", status)
+	}
+}