Refinements to git-merge-pull

- Be more prudent about not initiating rebase/merge process if the source and
  target branches don't match the corresponding remote branches.
- Perform the rebase on a separate <branch name>-rebase branch.
- Use colorize to make the output more pretty
- Change the merge commit message slightly.
- Generally be more explicit with git commands around remote and branch. Don't
  assume mapping is setup properly.
and passing :failure would be optional, so you could remove all the nil arguments below.