fix: resolve git repo root so revdiff works from subdirectories

CLAUDE.md

@@ -60,3 +60,4 @@ git diff → diff.ParseUnifiedDiff() → []DiffLine
 - Highlighted lines are pre-computed once per file load, stored parallel to `diffLines`
 - `DiffLine.Content` has no `+`/`-` prefix - prefix is re-added at render time
 - Tab replacement happens at render time in `renderDiffLine`, not in diff parsing
+- `run()` resolves git repo root via `git rev-parse --show-toplevel` so revdiff works from any subdirectory

cmd/revdiff/main.go

@@ -1,9 +1,11 @@
 package main
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"runtime"
 	"strings"
@@ -161,7 +163,11 @@ func defaultConfigPath() string {
 }
 
 func run(opts options) error {
-	renderer := diff.NewGit(".")
+	repoRoot, err := gitTopLevel()
+	if err != nil {
+		return fmt.Errorf("find git root: %w", err)
+	}
+	renderer := diff.NewGit(repoRoot)
 	store := annotation.NewStore()
 	hl := highlight.New(opts.ChromaStyle, !opts.NoColors)
 	model := ui.NewModel(renderer, store, hl, ui.ModelConfig{
@@ -216,3 +222,17 @@ func run(opts options) error {
 	fmt.Print(output)
 	return nil
 }
+
+// gitTopLevel returns the root directory of the current git repository.
+func gitTopLevel() (string, error) {
+	cmd := exec.CommandContext(context.Background(), "git", "rev-parse", "--show-toplevel")
+	out, err := cmd.Output()
+	if err != nil {
+		var exitErr *exec.ExitError
+		if errors.As(err, &exitErr) {
+			return "", fmt.Errorf("git rev-parse --show-toplevel: %s", strings.TrimSpace(string(exitErr.Stderr)))
+		}
+		return "", fmt.Errorf("git rev-parse --show-toplevel: %w", err)
+	}
+	return strings.TrimSpace(string(out)), nil
+}

cmd/revdiff/main_test.go

@@ -232,3 +232,19 @@ func TestDefaultConfigPath(t *testing.T) {
 	assert.Contains(t, path, "revdiff")
 	assert.Contains(t, path, "config")
 }
+
+func TestGitTopLevel(t *testing.T) {
+	t.Run("inside repo", func(t *testing.T) {
+		root, err := gitTopLevel()
+		require.NoError(t, err)
+		assert.DirExists(t, root)
+		assert.NotEmpty(t, root)
+	})
+
+	t.Run("outside repo", func(t *testing.T) {
+		t.Chdir(t.TempDir())
+		_, err := gitTopLevel()
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "git rev-parse --show-toplevel")
+	})
+}