Add switch branch (b): stage the tip and restore to it

b opens a centered popup with a search line and the local and remote
branches of the selection. Enter stages the branch tip as the pin of a
submodule, restores it (and everything below) to it, and puts HEAD on the
branch; a remote branch gets a local tracking branch unless one exists
elsewhere. Works on the superproject too. Losses are asked like restore.
The backdrop behind the popup is dimmed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/submodule/git/switch_test.go

@@ -0,0 +1,181 @@
+package git
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"submarine/internal/submodule"
+)
+
+func findBranch(t *testing.T, bs []submodule.Branch, name string) submodule.Branch {
+	t.Helper()
+	for _, b := range bs {
+		if b.Name == name {
+			return b
+		}
+	}
+	t.Fatalf("no branch %q in %v", name, bs)
+	return submodule.Branch{}
+}
+
+func TestBranchesListsLocalAndRemote(t *testing.T) {
+	p, root := ownFixture(t)
+	_ = root
+	bs, err := p.Branches(context.Background(), "libs/feature")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var names []string
+	seenRemote := false
+	for _, b := range bs {
+		names = append(names, b.Name)
+		if b.Remote {
+			seenRemote = true
+		} else if seenRemote {
+			t.Errorf("local branch %s after a remote one: %v", b.Name, names)
+		}
+		if b.Name == "origin/HEAD" {
+			t.Error("origin/HEAD must not be listed")
+		}
+	}
+	rb := findBranch(t, bs, "origin/feat/search")
+	if !rb.Remote || rb.Local != "feat/search" {
+		t.Errorf("origin/feat/search = %+v", rb)
+	}
+	findBranch(t, bs, "origin/main")
+}
+
+func TestSwitchSubmoduleStagesTipAndTracksRemoteBranch(t *testing.T) {
+	p, root := ownFixture(t)
+	super := filepath.Join(root, "super")
+	sub := filepath.Join(super, "libs", "feature")
+	ctx := context.Background()
+
+	bs, err := p.Branches(ctx, "libs/feature")
+	if err != nil {
+		t.Fatal(err)
+	}
+	b := findBranch(t, bs, "origin/feat/search")
+	got, err := p.Switch(ctx, "libs/feature", b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "feat/search" {
+		t.Errorf("Switch returned %q, want feat/search", got)
+	}
+	if head := out(t, sub, "branch", "--show-current"); head != "feat/search" {
+		t.Errorf("branch = %q", head)
+	}
+	if up := out(t, sub, "rev-parse", "--abbrev-ref", "feat/search@{upstream}"); up != "origin/feat/search" {
+		t.Errorf("upstream = %q", up)
+	}
+	if head := out(t, sub, "rev-parse", "HEAD"); head != b.SHA {
+		t.Errorf("HEAD = %s, want %s", head, b.SHA)
+	}
+	if staged := out(t, super, "ls-files", "-s", "--", "libs/feature"); !strings.Contains(staged, b.SHA) {
+		t.Errorf("index = %q, want the tip %s staged", staged, b.SHA)
+	}
+	assertExact(t, sub)
+}
+
+func TestSwitchNeverMovesAnExistingLocalBranch(t *testing.T) {
+	p, root := ownFixture(t)
+	sub := filepath.Join(root, "super", "libs", "feature")
+	ctx := context.Background()
+
+	git(t, sub, "branch", "feat/search", "origin/main") // same name, elsewhere
+	before := out(t, sub, "rev-parse", "feat/search")
+	bs, err := p.Branches(ctx, "libs/feature")
+	if err != nil {
+		t.Fatal(err)
+	}
+	b := findBranch(t, bs, "origin/feat/search")
+	got, err := p.Switch(ctx, "libs/feature", b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "" {
+		t.Errorf("Switch returned %q, want detached", got)
+	}
+	if head := out(t, sub, "branch", "--show-current"); head != "" {
+		t.Errorf("HEAD on %q, want detached", head)
+	}
+	if after := out(t, sub, "rev-parse", "feat/search"); after != before {
+		t.Errorf("branch moved: %s -> %s", before, after)
+	}
+	if head := out(t, sub, "rev-parse", "HEAD"); head != b.SHA {
+		t.Errorf("HEAD = %s, want %s", head, b.SHA)
+	}
+}
+
+func TestSwitchAbortsWhenTheBranchMoved(t *testing.T) {
+	p, root := ownFixture(t)
+	sub := filepath.Join(root, "super", "libs", "feature")
+	ctx := context.Background()
+
+	bs, err := p.Branches(ctx, "libs/feature")
+	if err != nil {
+		t.Fatal(err)
+	}
+	b := findBranch(t, bs, "origin/main")
+	b.SHA = out(t, sub, "rev-parse", "origin/feat/search") // what the list showed is stale
+	if _, err := p.Switch(ctx, "libs/feature", b); err == nil || !strings.Contains(err.Error(), "moved") {
+		t.Errorf("err = %v, want a moved error", err)
+	}
+}
+
+func TestSwitchSuperprojectRestoresSubmodulesToItsPins(t *testing.T) {
+	p, root := ownFixture(t)
+	super := filepath.Join(root, "super")
+	ctx := context.Background()
+
+	git(t, super, "branch", "older", "HEAD~1")
+	bs, err := p.Branches(ctx, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	b := findBranch(t, bs, "older")
+	got, err := p.Switch(ctx, "", b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "older" || out(t, super, "branch", "--show-current") != "older" {
+		t.Errorf("Switch = %q, branch = %q", got, out(t, super, "branch", "--show-current"))
+	}
+	assertExact(t, super)
+}
+
+func TestSwitchLossCountsChangedFiles(t *testing.T) {
+	p, root := ownFixture(t)
+	sub := filepath.Join(root, "super", "libs", "feature")
+	ctx := context.Background()
+
+	bs, err := p.Branches(ctx, "libs/feature")
+	if err != nil {
+		t.Fatal(err)
+	}
+	b := findBranch(t, bs, "origin/feat/search")
+	loss, err := p.SwitchLoss(ctx, "libs/feature", b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !loss.Empty() {
+		t.Errorf("clean checkout: loss = %+v", loss)
+	}
+	if err := os.WriteFile(filepath.Join(sub, "new.txt"), []byte("x"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if loss, err = p.SwitchLoss(ctx, "libs/feature", b); err != nil || loss.Files != 1 {
+		t.Errorf("loss = %+v, %v; want 1 file", loss, err)
+	}
+}
+
+func TestBranchesOfAMissingSubmoduleSaysSo(t *testing.T) {
+	p, _ := ownFixture(t)
+	if _, err := p.Branches(context.Background(), "libs/missing"); err == nil || !strings.Contains(err.Error(), "not initialized") {
+		t.Errorf("err = %v", err)
+	}
+}