SPARK-3648: Provide a script for fetching remote PR's for review

This is a simple script I wrote for fetching remote pull requests.
I found this very useful during code reviews and I think other
developers would as well.

One nice thing is this allows you to avoid creating local refs for
all pull requests, which can be annoying since there is high churn
in PR activity.
It turns out that I have multiple remotes that point to the Apache Spark Github:

```
git remote -v |grep apache/spark\.git | grep fetch
apache-github	git://github.com/apache/spark.git (fetch)
origin	git@github.com:apache/spark.git (fetch)
```

This caused the checkout to fail:

```
./dev/fetch-pr 3000
Detected Apache github remote 'apache-github
origin'
Fetching remote ref 'pull/3000/head'
fatal: Couldn't find remote ref origin
```