Update index before diff-index to check if there are changes

[adamtheturtle]

Thank you @asottile for the tooling and apologies for the earlier spamming.

It is possible to get to a state where diff-index shows that there is a diff, but there is nothing to commit.
For example, when overwriting a file with its current context.

Experiment:

$ echo hello > foo.txt
$ git add foo.txt
$ git diff-index --no-ext-diff HEAD --
:000000 100644 0000000000000000000000000000000000000000 ce013625030ba8dba906f756967f9e9ca394464a A      foo.txt
$ git commit -m "Add foo.txt"
[experiment b237cf0] Add foo.txt
 1 file changed, 1 insertion(+)
 create mode 100644 foo.txt
$ git diff-index --no-ext-diff HEAD --
$ echo hello > foo.txt
$ git diff-index --no-ext-diff HEAD --
:100644 100644 ce013625030ba8dba906f756967f9e9ca394464a 0000000000000000000000000000000000000000 M      foo.txt
$ git commit -a
On branch experiment
nothing to commit, working tree clean
$

This scenario led to a confusing error:

Traceback (most recent call last):
  File "/home/runner/work/_actions/pre-commit-ci/lite-action/v1.0.2/bin/main", line 298, in <module>
    raise SystemExit(main())
                     ^^^^^^
  File "/home/runner/work/_actions/pre-commit-ci/lite-action/v1.0.2/bin/main", line 270, in main
    commit = _make_commit(src_repo=args.src_repo, head=head, clone=clone)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/_actions/pre-commit-ci/lite-action/v1.0.2/bin/main", line 57, in _make_commit
    subprocess.check_call(
  File "/opt/hostedtoolcache/Python/3.12.5/x64/lib/python3.12/subprocess.py", line 413, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '('git', '-c', 'user.name=does-not-matter', '-c', '[email protected]', '-c', 'protocol.version=2', '-C', '.', 'commit', '--all', '--quiet', '--no-edit', '--no-verify', '--message=hi')' returned non-zero exit status 1.

This change refreshes the index before diff-ing it, avoiding the error.

[asottile]