Error when generating files for pretty diff
When generating pretty diffs a `Failed...` error message pops up, and no commit on the repository is performed.
### Steps to reproduce
#### Commiting
- Edit/add a sketch file in a git repository
- Show commit prompt via `Plugins > Git > Commit`
- `Generate files for pretty diffs` is ticked
- Enter commit message, hit `Commit``
##### Direct Invocation
- `Plugins > Git > Generate files for pretty diffs`
### Error message:

``` log
*** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: 
attempt to insert nil object from objects[3]
((null) ECCommandLineDomain:5)
```
### Used versions:

```
❯ chest list
(git)   git-sketch-plugin bb101a3bbc77947bb7c6890b5c4600b962b43bf9

❯ sketchtool -v
sketchtool Version 3.5.2 (25235)
```
