Add tools/scripts/merge-forward

Add script for merging changes from the maintenance branch (4.3) to the main development branch (master). Examples:

```
./tools/scripts/merge-forward packages upstream 4.3 master
./tools/scripts/merge-forward joomla upstream 4.3 master    
./tools/scripts/merge-forward WordPress upstream 4.3 master    
./tools/scripts/merge-forward drupal upstream 7.x-4.3 7.x-master    
./tools/scripts/merge-forward drupal upstream 6.x-4.3 6.x-master    
./tools/scripts/merge-forward . upstream 4.3 master    
```

If there are no conflicts or no changes, then the script is automatic. If there are conflicts, then they must be resolved and the new changes pushed.

(Note: I also played around with integrating this functionality into givi so that all repos could be merged with a single command. It proved difficult to provide a developer-experience that was easy to use/understand -- i.e. putting merge-forward into givi required either a lot of code or a lot of questions/options for the developer. The current approach requires a few more commands but should feel trivial in the usual cases and comprehensible in the hard cases.)