move git executions from PullRequestSwitchBaseCommand
improve switching pull-request base command

- Switch base in active pull request (when supported by adapter)
  else create a new PR with the same information (but new base)
- Inform if a new PR was made during the base-switch
- Abort git rebase process on failure
move git executions from PullRequestSquashCommand

As a little bonus this will preserve the original first-subject