Git::Diff - don't convert to empty string if from/to is nil

I was having trouble just getting the code to do a git diff without specifying commits.  

```
I, [2014-09-26T16:21:18.444662 #65864]  INFO -- : git diff '-p' 'HEAD' ''  2>&1
D, [2014-09-26T16:21:18.444727 #65864] DEBUG -- : fatal: ambiguous argument '': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
/Users/fluong/.rvm/gems/ruby-2.0.0-p353/gems/git-1.2.8/lib/git/lib.rb:764:in `command': git diff '-p' 'HEAD' ''  2>&1:fatal: ambiguous argument '': unknown revision or path not in the working tree. (Git::GitExecuteError)
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
	from /Users/fluong/.rvm/gems/ruby-2.0.0-p353/gems/git-1.2.8/lib/git/lib.rb:276:in `diff_full'
	from /Users/fluong/.rvm/gems/ruby-2.0.0-p353/gems/git-1.2.8/lib/git/diff.rb:100:in `cache_full'
	from /Users/fluong/.rvm/gems/ruby-2.0.0-p353/gems/git-1.2.8/lib/git/diff.rb:51:in `patch'
	from ./git-diff.rb:9:in `<main>'
```

I think this will fix it.