Add --continue option to rebase
Use full sha1 on the command-line

Rebasing openmicroscopy/openmicroscopy#641
failed with

```
2013-01-25 09:04:51,449 [     scc.git] INFO  Branching SHA1: f70c33
2013-01-25 09:04:51,528 [     scc.git] INFO  error: short SHA1 f70c33 is ambiguous.
2013-01-25 09:04:51,529 [     scc.git] INFO  fatal: Needed a single revision
2013-01-25 09:04:51,550 [     scc.git] INFO  invalid upstream f70c33
```
Merge pull request #55 from joshmoore/rebase-restart

Add --continue option to rebase