fix README
Fix delete patchset

Delete models.Patch that have a FK to the patchset being deleted.
This FK check caused mysql to spew an error whenever a user attempted to
a delete a patchset.
Add '--no-index' switch to git-diff call

--no-index forces git to treat the two paths as two files / directories
From the man page:

git diff [--options] [--] [<path>...]
   This form is to view the changes you made relative to the index (staging area for the next commit). In other words, the differences are what you could tell git to further add to the
   index but you still haven’t. You can stage these changes by using git-add(1).

   If exactly two paths are given and at least one points outside the current repository, git diff will compare the two files / directories. This behavior can be forced by --no-index.