UnrebasedPRs command

This command allows the interrogation of two branches to see
which PRs on one branch have not yet been rebased onto the other.
A typical workflow includes:

```
  scc unrebased-prs dev_4_4 develop          # See a listing
  scc unrebased-prs dev_4_4 develop --write  # Write the listing to 2 files
  mvim -d *prs.txt                           # Edit the files in diff mode.
                                             # Make sure all lines match.
  scc unrebased-prs dev_4_4 develop --parse  # Parse the files into commands
  rm *prs.txt                                # Remove the pr files.
  git push origin refs/notes/*               # Push the notes to origin
```