feat: allow to select branch for pull request
feat: allow to select branch for pull request

Enables the selection of a branch during creation of a pull request.
To use this feature the command 'Create pull request...' must be
executed and after the repository selection the branch selection is
shown. The branch shown by default is the github default branch of the
selected repository.

Closes #125

KnisterPeter/vscode-github,KnisterPeter/vscode-github