refactor(syncer): detect repos with os.Stat to support git worktrees

IsRepo previously called os.ReadDir(dirName) and walked every entry
looking for a name match against ".git". That has two problems:
it pays full directory-listing cost just to answer a yes/no question
about a single fixed name, and the name-only match doesn't make the
worktree case explicit even though git worktrees place ".git" as a
*file* (a "gitdir: ..." pointer) rather than a directory.

Replace the loop with a single os.Stat on the joined path. One
syscall instead of N, and stat doesn't care whether the entry is a
file or a directory, so a worktree's .git file is correctly accepted
as a repository indicator.

Add syncer/dir_test.go covering both layouts plus the negative cases
(non-git directory, non-existent path), pinning the worktree-support
contract so a future "must be a directory" change would surface as a
test failure.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: takecy

syncer/dir.go

@@ -2,6 +2,7 @@ package syncer
 
 import (
 	"os"
+	"path/filepath"
 	"strings"
 )
 
@@ -22,18 +23,11 @@ func ListDirs() (dirs []string, err error) {
 	return
 }
 
-// IsRepo returns check result, the directory whether git repository
+// IsRepo reports whether dirName contains a `.git` entry. It accepts both a
+// regular repository (where `.git` is a directory) and a git worktree
+// (where `.git` is a file pointing at the main repo's git directory),
+// because exec'd git commands work in either layout.
 func IsRepo(dirName string) bool {
-	files, err := os.ReadDir(dirName)
-	if err != nil {
-		return false
-	}
-
-	for _, f := range files {
-		if f.Name() == ".git" {
-			return true
-		}
-	}
-
-	return false
+	_, err := os.Stat(filepath.Join(dirName, ".git"))
+	return err == nil
 }

syncer/dir_test.go

@@ -0,0 +1,64 @@
+package syncer
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/matryer/is"
+)
+
+func TestIsRepo(t *testing.T) {
+	t.Run("regular repo with .git directory", func(t *testing.T) {
+		t.Parallel()
+		is := is.New(t)
+
+		dir := t.TempDir()
+		repo := filepath.Join(dir, "myrepo")
+		if err := os.MkdirAll(filepath.Join(repo, ".git"), 0o755); err != nil {
+			t.Fatalf("setup: %v", err)
+		}
+
+		is.True(IsRepo(repo))
+	})
+
+	t.Run("worktree with .git file", func(t *testing.T) {
+		t.Parallel()
+		is := is.New(t)
+
+		dir := t.TempDir()
+		worktree := filepath.Join(dir, "myworktree")
+		if err := os.MkdirAll(worktree, 0o755); err != nil {
+			t.Fatalf("setup: %v", err)
+		}
+		// git worktrees place a .git file (not directory) containing
+		// a `gitdir: <path>` pointer. The exact contents don't matter for
+		// IsRepo — only that the entry exists.
+		gitFile := filepath.Join(worktree, ".git")
+		if err := os.WriteFile(gitFile, []byte("gitdir: /tmp/main/.git/worktrees/x\n"), 0o644); err != nil {
+			t.Fatalf("setup: %v", err)
+		}
+
+		is.True(IsRepo(worktree))
+	})
+
+	t.Run("non-git directory returns false", func(t *testing.T) {
+		t.Parallel()
+		is := is.New(t)
+
+		dir := t.TempDir()
+		plain := filepath.Join(dir, "plain")
+		if err := os.MkdirAll(plain, 0o755); err != nil {
+			t.Fatalf("setup: %v", err)
+		}
+
+		is.True(!IsRepo(plain))
+	})
+
+	t.Run("non-existent directory returns false", func(t *testing.T) {
+		t.Parallel()
+		is := is.New(t)
+
+		is.True(!IsRepo(filepath.Join(t.TempDir(), "does-not-exist")))
+	})
+}