Fix `git-clang-format <commit1> <commit2>`.

[lodato]

Summary

This feature had never worked at all because the code incorrectly used <commit2> for both the "old" and "new", so the diff was always empty! Now we correctly use <commit1> where we should and it works fine.

[lodato]

Actually the old code might have been working fine. Please hold off on reviewing.

[lodato]

Yeah, I was incorrect. The feature worked fine. I don't know what I was thinking.

google3/third_party/llvm/llvm/tools/clang/tools/clang-format/git-clang-format

if changed_lines:
         print('    %s' % filename)
   if not changed_lines:
     print('no modified files to format')
     return
   # The computed diff outputs absolute paths, so we must cd before accessing
   # those files.
   cd_to_toplevel()
   if len(commits) > 1:
-    old_tree = commits[1]
+    old_tree = commits[0]
     new_tree = run_clang_format_and_save_to_tree(changed_lines,
                                                  revision=commits[1],
                                                  binary=opts.binary,
                                                  style=opts.style)
   else:
     old_tree = create_tree_from_workdir(changed_lines)
     new_tree = run_clang_format_and_save_to_tree(changed_lines,
                                                  binary=opts.binary,