vimdiff as mergetool causes E810 and E97

I am using version `2.6.0.windows.1`, this also occurred in `2.5.3.windows.1`.

Relevant snippet from `.gitconfig`:

```
[merge]
    tool = vimdiff
    conflictstyle = diff3
[mergetool]
    prompt = false
```

When I run `git mergetool` I get this error:

```
E810: Cannot read or write temp files
E97: Cannot create diffs
Press ENTER or type command to continue
```
