[Bitbucket]: Create new PR with remote doesn't list remote branches

[jeremyfiel]

Describe the bug
Create a PR with upstream remote, the dropdown list of source branches is not populated correctly.

To Reproduce

1. Add an upstream remote to your repo
2. Create a new PR with the extension
3. The dropdown should show your origin and upstream and the list of branches for the default list selection, in this case, origin
4. Change the Remote to upstream and the Source Branch dropdown does not update with the correct branch list
5. No debug output is available.

Expected behavior
I want to see the list of branches available on the remote

Screenshots

origin

remote

Environment and Version (please complete the following information):

• Atlassian Plugin Version (3.4.9)
• Visual Studio Code Version (x.x.x)
• Is this a local or cloud environment? (Cloud | Local)
• If Cloud, Browser (chrome, safari)
• If Local, OS (macOS, windows, linux)

Additional context
Add any other context about the problem here.

[jeremyfiel]

possibly related #139