support comma-separated fetch specs list?

From: ryenus <ryenus@gmail.com>
Date: Nov 4, 2020, 03:11 UTC
Message-ID: <CAKkAvaxrG0aYw-wo_WW7zSkX=mjU8ABgXGbbPCFOhMBa4WTJ+Q@mail.gmail.com>

It's nice to have negative ref spec support with 2.29 release, so now we can have

```
[remote "origin"]
  fetch = +refs/heads/*:refs/remotes/origin/*
  fetch = ^refs/heads/gh-pages
```

What about having all the fetch specs put together in one comma-separated list? Like this:

```
[remote "origin"]
  fetch = +refs/heads/*:refs/remotes/origin/*,^refs/heads/gh-pages
```

Thanks