fix(show-index): adapt stdin reader to idxfile decoder input API

Agent-Logs-Url: https://github.com/go-git/cli/sessions/fdb5eddf-67f4-4606-869c-ae677df53b81

Co-authored-by: pjbgf <5452977+pjbgf@users.noreply.github.com>

Author: Copilot

cmd/gogit/show-index.go

@@ -1,10 +1,13 @@
 package main
 
 import (
+	"bytes"
 	"crypto"
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
+	"time"
 
 	"github.com/go-git/go-git/v6/plumbing/format/idxfile"
 	"github.com/go-git/go-git/v6/plumbing/hash"
@@ -29,7 +32,12 @@ var showIndexCmd = &cobra.Command{
 
 func showIndexRun(in io.Reader, out io.Writer) error {
 	idx := idxfile.NewMemoryIndex(crypto.SHA1.Size())
-	dec := idxfile.NewDecoder(in, hash.New(crypto.SHA1))
+	idxIn, err := idxInput(in)
+	if err != nil {
+		return err
+	}
+
+	dec := idxfile.NewDecoder(idxIn, hash.New(crypto.SHA1))
 
 	if err := dec.Decode(idx); err != nil {
 		return fmt.Errorf("decode idx: %w", err)
@@ -56,3 +64,35 @@ func showIndexRun(in io.Reader, out io.Writer) error {
 
 	return nil
 }
+
+func idxInput(in io.Reader) (idxfile.Input, error) {
+	b, err := io.ReadAll(in)
+	if err != nil {
+		return nil, fmt.Errorf("read idx input: %w", err)
+	}
+
+	return &memoryIdxInput{
+		Reader: bytes.NewReader(b),
+		size:   int64(len(b)),
+	}, nil
+}
+
+type memoryIdxInput struct {
+	*bytes.Reader
+	size int64
+}
+
+func (in *memoryIdxInput) Stat() (fs.FileInfo, error) {
+	return memoryIdxFileInfo{size: in.size}, nil
+}
+
+type memoryIdxFileInfo struct {
+	size int64
+}
+
+func (fi memoryIdxFileInfo) Name() string       { return "" }
+func (fi memoryIdxFileInfo) Size() int64        { return fi.size }
+func (fi memoryIdxFileInfo) Mode() fs.FileMode  { return 0 }
+func (fi memoryIdxFileInfo) ModTime() time.Time { return time.Time{} }
+func (fi memoryIdxFileInfo) IsDir() bool        { return false }
+func (fi memoryIdxFileInfo) Sys() any           { return nil }