*: add an shell script to make checkout PR branch easier. (pingcap#4794)

When a PR is not trivial, we need to checkout the branch to review the PR, this scripts makes this task easier.

checkout-pr-branch.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+# This script is used to checkout a TiDB PR branch in a forked repo.
+if test -z $1; then
+	echo -e "Usage:\n"
+	echo -e "\tcheckout-pr-branch.sh [github-username]:[pr-branch]\n"
+	echo -e "The argument can be copied directly from github PR page."
+	echo -e "The local branch name would be [github-username]/[pr-branch]."
+	exit 0;
+fi
+
+username=$(echo $1 | cut -d':' -f1)
+branch=$(echo $1 | cut -d':' -f2)
+local_branch=$username/$branch
+fork="https://github.com/$username/tidb"
+git fetch $fork $branch:$local_branch
+git checkout $local_branch