Don't fall back to pwd for Git dir of blank filename

In particular, this fixes `FugitiveFind(..., '')` to respect the empty
Git dir argument rather than falling back to detection on the current
working directory.  Which in turn fixes :Gstatus using the current
working directory when called from a buffer that does not belong to a
repository.

References https://github.com/tpope/vim-fugitive/issues/1408

plugin/fugitive.vim

@@ -181,6 +181,8 @@ function! FugitiveExtractGitDir(path) abort
   let path = s:Slash(a:path)
   if path =~# '^fugitive:'
     return matchstr(path, '\C^fugitive:\%(//\)\=\zs.\{-\}\ze\%(//\|::\|$\)')
+  elseif empty(path)
+    return ''
   elseif isdirectory(path)
     let path = fnamemodify(path, ':p:s?/$??')
   else