Provide FugitiveActualDir()

This is intended to free up FugitiveGitDir() to refer to the ".git" file
in that work tree and submodule repositories use.  I wish I had better
names for these two.  Settling for "Actual" as that at least has a
pleasing 6 letter symmetry with "Common".

plugin/fugitive.vim

@@ -283,9 +283,13 @@ function! FugitiveStatusline(...) abort
   return fugitive#Statusline()
 endfunction
 
+function! FugitiveActualDir(...) abort
+  return call('FugitiveGitDir', a:000)
+endfunction
+
 let s:commondirs = {}
 function! FugitiveCommonDir(...) abort
-  let dir = FugitiveGitDir(a:0 ? a:1 : -1)
+  let dir = call('FugitiveActualDir', a:000)
   if empty(dir)
     return ''
   endif