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

**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.

Reproduce the issue through the extension's Create a new PR flow with origin and upstream remotes, then compare the Source Branch dropdown after changing Remote. Done means the dropdown lists the branches available on the selected remote and behaves consistently for origin and upstream.

Written by the indexing model from the issue text.