don't process fugitive files

Closes #4.

plugin/signify.vim

@@ -245,6 +245,9 @@ function! s:diff_get(path) abort
 
   if executable('git')
     let orig_dir = getcwd()
+    if a:path =~ '^fugitive'
+      return ''
+    endif
     exe 'cd '. fnamemodify(a:path, ':h')
     let diff = system('git diff --no-ext-diff -U0 -- '. a:path .' | grep "^@@ "')
     if !v:shell_error
@@ -297,7 +300,7 @@ function! s:diff_get(path) abort
     endif
   endif
 
-  return []
+  return ''
 endfunction
 
 "  Functions -> s:diff_process()  {{{2