[clang-format] Fix a serious bug in git-clang-format (#65723)

When applying format changes to staged files, git-clang-format
erroneously checks out all files in the index and thus may overwrite
unstaged changes.

Fixes #65643.

(cherry picked from commit 743659b)

clang/tools/clang-format/git-clang-format

@@ -605,7 +605,7 @@ def apply_changes(old_tree, new_tree, force=False, patch_mode=False):
     index_tree = old_tree
   else:
     with temporary_index_file(new_tree):
-      run('git', 'checkout-index', '-a', '-f')
+      run('git', 'checkout-index', '-f', '--', *changed_files)
   return changed_files