Revert "Avoid IO from ":p" during detection"

This change failed to account for the directory edge case, where ":p:h"
has the very nice property of giving us back the original input, due to
":p" appending a trailing slash.  Without this, we fail to detect when
editing the root of the repository.

This reverts commit 1b811b88a4.

Resolves: https://github.com/tpope/vim-fugitive/issues/1978
Resolves: https://github.com/tpope/vim-fugitive/issues/1979

plugin/fugitive.vim

@@ -413,10 +413,9 @@ function! FugitiveExtractGitDir(path) abort
     return matchstr(path, '\C^fugitive:\%(//\)\=\zs.\{-\}\ze\%(//\|::\|$\)')
   elseif empty(path)
     return ''
-  elseif path !~# '^/\|^\a\+:'
-    let path = s:Slash(getcwd()) . '/' . path
+  else
+    let path = fnamemodify(path, ':p:h')
   endif
-  let path = fnamemodify(path, ':h')
   let pre = substitute(matchstr(path, '^\a\a\+\ze:'), '^.', '\u&', '')
   if len(pre) && exists('*' . pre . 'Real')
     let path ={pre}Real(path)