Dynamic read g:signify_diffoptions before each diff

Wanted to be able to diff against a different branch in git, but realized that this was not possible to do without changing my vimrc file, since signify-diffoptions is only looked at once.  

The change makes it so it is read from the global g:signify-diffoptions each time before diff is called.  This allows for the following use case (get applied next time diff is done, such as after a :SignifyToggle)
    # Show diff against master instead of against HEAD in Git
    nnoremap \u :let g:signify_diffoptions = { 'git': 'master' }<CR> 

This change will also allow any other arbitrary diff options to be dynamically updated, such as whitespace, etc.