git does not wait on hook exes (^C)

[Anthony Sottile <asottile@xxxxxxxxx>]

Here's a small example:

```bash
#!/usr/bin/env bash
foo() {
    echo zzz part 2
    sleep 1
    echo exiting now
}
trap foo SIGINT

echo zzz part 1
sleep 10
```

```console
$ git --version
git version 2.25.GIT
$ git commit --allow-empty -m foo
zzz part 1
^Czzz part 2

$ exiting now
```

- I pressed ^C during the first sleep
- control was returned back to my terminal
- the hook script was still running in the background

Anthony