Redefine rebase process

[andreisilviudragnea]

• Rebase open PRs for all remotes (including origin) originating from origin remote
• Redefine rebase process to work also across remotes (pull request from origin remote to upstream remote)
• Redefine safe PRs:
  • if head branch is checked out locally, it is equal to remote tracking branch; otherwise head branch is safe to check out locally
  • if base branch is checked out locally, it is equal to remote tracking branch; otherwise base branch is safe; or use base branch exactly from remote, but be careful when rebasing a branch.