Fix gb -D expansion

Here is a simple pseudo testcase:
Before:
```shell
~/work/test% gb                                    
* [1] master
~/work/test% gco -b test
Switched to a new branch 'test'
~/work/test% gb
  [1] master
* [2] test
~/work/test% gco 1
Switched to branch 'master'
~/work/test% gb -D 2
error: branch '2' not found.
```
After:
```shell
~/work/test% gb                                    
* [1] master
~/work/test% gco -b test
Switched to a new branch 'test'
~/work/test% gb
  [1] master
* [2] test
~/work/test% gco 1
Switched to branch 'master'
~/work/test% gb -D 2
Deleted branch test (was 8c5a00b).
```