git work pr update

[mdbs99]

Let's create a new command option: pr update
This command should update the current pr.
When the contributor updates his PR and it wasn't merged into master yet, we need to update the local pr to continue working.

The commands should be:

git fetch origin pull/<id>/head
git merge pull/<id>/head

The script needs to check if the current branch is a PR by prefix pr. Otherwise, the user gets an error.