Fix paths match for pull request create screen to handle merging and rebasing correctly

We should stream the changes using com.atlassian.bitbucket.compare.CompareService

We can't use the same way we do for when the pull request is created as the id doesn't exist.

It seems just getting the changes from the commit service does not give the required behaviour.

There should also be additional tests to cover the scenarios where a pull request is not yet created.