fix(diff,tool): read file content at reviewed ref in range/commit mode

file_find and finalizeDiff always read from the working tree, even in
range/commit mode where the review targets a specific git ref. This
caused inconsistent file versions compared to file_read and code_search
which correctly used git show. Fix file_find to use git ls-tree and
finalizeDiff to use git show when a ref is specified.

internal/diff/git.go

@@ -139,7 +139,15 @@ func (p *Provider) GetDiff() ([]model.Diff, error) {
 		}
 	}
 
-	diffs, err := ParseDiffText(combined.String(), p.repoDir)
+	var ref string
+	switch p.mode {
+	case ModeRange:
+		ref = p.to
+	case ModeCommit:
+		ref = p.commit
+	}
+
+	diffs, err := ParseDiffText(combined.String(), p.repoDir, ref)
 	if err != nil {
 		return nil, err
 	}

internal/diff/parser.go

@@ -2,11 +2,14 @@
 package diff
 
 import (
+	"context"
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"regexp"
 	"strings"
+	"time"
 
 	"github.com/open-code-review/open-code-review/internal/model"
 )
@@ -19,18 +22,23 @@ var (
 )
 
 // ParseDiffText splits the unified diff text into per-file Diff structs.
-func ParseDiffText(diffText string, repoDir string) ([]model.Diff, error) {
+// ref, if non-empty, is a git ref used to read new-file content via
+// git show instead of reading from the working tree.
+func ParseDiffText(diffText string, repoDir string, ref string) ([]model.Diff, error) {
 	lines := strings.Split(diffText, "\n")
 	var diffs []model.Diff
 	var current *model.Diff
 	var buf strings.Builder
 
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
+	defer cancel()
+
 	for _, line := range lines {
 		if m := diffHeaderRe.FindStringSubmatch(line); m != nil {
 			// Flush previous diff
 			if current != nil {
 				current.Diff = strings.TrimSuffix(buf.String(), "\n")
-				finalizeDiff(current, repoDir)
+				finalizeDiff(ctx, current, repoDir, ref)
 				diffs = append(diffs, *current)
 				buf.Reset()
 			}
@@ -66,19 +74,33 @@ func ParseDiffText(diffText string, repoDir string) ([]model.Diff, error) {
 	// Flush last diff
 	if current != nil {
 		current.Diff = strings.TrimSuffix(buf.String(), "\n")
-		finalizeDiff(current, repoDir)
+		finalizeDiff(ctx, current, repoDir, ref)
 		diffs = append(diffs, *current)
 	}
 
 	return diffs, nil
 }
 
-// finalizeDiff attempts to read the new file content from disk.
-func finalizeDiff(d *model.Diff, repoDir string) {
+// finalizeDiff reads the new file content. When ref is non-empty it uses
+// git show to read the file at that ref; otherwise it reads from disk.
+func finalizeDiff(ctx context.Context, d *model.Diff, repoDir string, ref string) {
 	if d.IsDeleted || d.NewPath == "/dev/null" {
 		d.NewPath = "/dev/null"
 		return
 	}
+	if ref != "" {
+		cmd := exec.CommandContext(ctx, "git", "-c", "core.quotepath=false",
+			"show", ref+":"+d.NewPath)
+		cmd.Dir = repoDir
+		output, err := cmd.Output()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "[ocr] WARNING: cannot read file %s at ref %s: %v\n",
+				d.NewPath, ref, err)
+			return
+		}
+		d.NewFileContent = string(output)
+		return
+	}
 	fullPath := filepath.Join(repoDir, d.NewPath)
 	content, err := os.ReadFile(fullPath)
 	if err != nil {

internal/tool/file_find.go

@@ -57,8 +57,14 @@ func (p *FileFindProvider) Execute(args map[string]any) (string, error) {
 }
 
 // listGitFiles returns tracked and untracked files (respecting .gitignore) via git ls-files.
+// In range/commit mode it uses git ls-tree to list files at the reviewed ref.
 func (p *FileFindProvider) listGitFiles() ([]string, error) {
-	cmd := exec.Command("git", "ls-files", "--cached", "--others", "--exclude-standard")
+	var cmd *exec.Cmd
+	if ref := p.FileReader.Ref; ref != "" {
+		cmd = exec.Command("git", "ls-tree", "-r", "--name-only", ref)
+	} else {
+		cmd = exec.Command("git", "ls-files", "--cached", "--others", "--exclude-standard")
+	}
 	cmd.Dir = p.FileReader.RepoDir
 	output, err := cmd.Output()
 	if err != nil {