4.8.0 Add `--sourceBranch` option to backport from non-master branches

By default the list of commits will be sourced from the repository's default branch (mostly "master"). sourceBranch can be used to list and backport commits from other branches than the default.

Configuration:

// Example .backportrc.json
{
  "sourceBranch": "7.x"
}

CLI: --sourceBranch 7.x