line 80: git-diff: command not found

I went through following steps before I got the `git-diff` command not found error.

```
# curl https://raw.github.com/philz/git-vimdiff/master/git-vimdiff > /usr/local/bin/git-vimdiff
# chmod ugo+x /usr/local/bin/git-vimdiff
```

After firing `git-vimdiff`, I get following eror 

```
    # git-vimdiff 
    /usr/local/bin/git-vimdiff: line 80: git-diff: command not found
```
