refactor/gitindex: use maps.Keys for deterministic config key ordering

The helper that sorted config keys manually built an intermediate slice before sorting, even though Go now provides an iterator-first path for this pattern.

Using maps.Keys with slices.Sorted keeps the helper shorter and makes the intent explicit while preserving deterministic ordering for clone and config-update command generation.

Test Plan: go test ./...

Amp-Thread-ID: https://ampcode.com/threads/T-019d0be8-9ecb-7004-acaa-a53513692639
Co-authored-by: Amp <amp@ampcode.com>

Author: keegancsmith

gitindex/clone_config.go

@@ -18,18 +18,14 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"maps"
 	"os/exec"
-	"sort"
+	"slices"
 	"strings"
 )
 
 func sortedKeys(settings map[string]string) []string {
-	keys := make([]string, 0, len(settings))
-	for k := range settings {
-		keys = append(keys, k)
-	}
-	sort.Strings(keys)
-	return keys
+	return slices.Sorted(maps.Keys(settings))
 }
 
 func cloneConfigArgs(settings map[string]string) []string {