Fetch in the background and follow changes in the repositories

- On startup, after the local state is shown, fetch the superproject and
  every initialized submodule (all remotes, prune, at most 8 at once).
  Fetches never prompt; failures show git's message. --no-fetch skips it,
  f fetches again.
- The header shows "fetching n/m", then the fetch age and failures; the
  detail pane shows each submodule's last fetch and fetch errors.
- The git provider polls each repository's metadata (refs, HEAD, index,
  .gitmodules) every 2 s and reports changes. The app reloads only what
  changed, keeping old values, expansion and cursor; loads are
  single-flight with coalesced reruns. Refresh is driven by the
  repositories, so a fetch in another shell updates the view like ours.
- r reloads everything (worktree edits are not watched).
- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).

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

internal/submodule/git/fetch.go

@@ -0,0 +1,79 @@
+package git
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+	"time"
+)
+
+// fetchTimeout bounds a single fetch, e.g. against an unreachable host.
+const fetchTimeout = 2 * time.Minute
+
+// maxFetches bounds concurrent fetches; they wait on the network, not the
+// CPU.
+const maxFetches = 8
+
+// sshBatchEnv makes ssh fail instead of asking for a password or host key,
+// which would corrupt the terminal UI. It is nil if the user configured
+// their own ssh command.
+func sshBatchEnv(root string) []string {
+	if os.Getenv("GIT_SSH_COMMAND") != "" {
+		return nil
+	}
+	if out, ok, _ := try(context.Background(), root, "config", "core.sshCommand"); ok && out != "" {
+		return nil
+	}
+	return []string{"GIT_SSH_COMMAND=ssh -o BatchMode=yes"}
+}
+
+// Fetchable reports whether path ("" is the superproject) is a checked-out
+// repository.
+func (p *Provider) Fetchable(path string) bool {
+	_, ok := p.repoDir(path)
+	return ok
+}
+
+// repoDir returns the worktree of path if it is initialized.
+func (p *Provider) repoDir(path string) (string, bool) {
+	if path == "" {
+		return p.root, true
+	}
+	p.mu.RLock()
+	in, ok := p.mods[path]
+	p.mu.RUnlock()
+	if !ok || !initialized(in.dir) {
+		return "", false
+	}
+	return in.dir, true
+}
+
+// Fetch fetches all remotes of one repository. Submodules are fetched
+// separately, not recursively. The error is git's first line of output.
+func (p *Provider) Fetch(ctx context.Context, path string) error {
+	dir, ok := p.repoDir(path)
+	if !ok {
+		return fmt.Errorf("%s is not initialized", path)
+	}
+	select {
+	case p.fetchSem <- struct{}{}:
+		defer func() { <-p.fetchSem }()
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+	ctx, cancel := context.WithTimeout(ctx, fetchTimeout)
+	defer cancel()
+
+	_, err := runEnv(ctx, dir, p.sshEnv, "fetch", "--all", "--prune", "--no-recurse-submodules", "--quiet")
+	var ge *gitError
+	switch {
+	case errors.Is(err, context.DeadlineExceeded):
+		return fmt.Errorf("timed out after %s", fetchTimeout)
+	case errors.As(err, &ge) && ge.Stderr != "":
+		line, _, _ := strings.Cut(ge.Stderr, "\n")
+		return errors.New(strings.TrimPrefix(line, "fatal: "))
+	}
+	return err
+}