Fix omitted Git dir argument

This shouldn't matter in this context since it's designed only to
provoke an error message, but fix it anyways so it stops setting off
mental alarm bells.

autoload/fugitive.vim

@@ -5893,7 +5893,7 @@ function! s:BlameSubcommand(line1, count, range, bang, mods, options) abort
       let i += 1
       if i == len(flags)
         echohl ErrorMsg
-        echo s:ChompError(['blame', arg])[0]
+        echo s:ChompError([dir, 'blame', arg])[0]
         echohl NONE
         return ''
       endif