allow for making diff from the current working directory to HEAD


I wanted to get a patch with the changes I had in my working directory from HEAD. 
It turns out that calling nil.to_s  is causing to create a command like:
git diff "-p" "HEAD" ""  2>&1

This small change makes the nil survive till the where the command is created and it becomes
git diff "-p" "HEAD"  2>&1
which works as expected.