Sync up win32 and UNIX fugitive:// regexps

Remove support for an optional second slash delimiter that was
accidentally committed in the previous win32 change.

autoload/fugitive.vim

@@ -137,7 +137,7 @@ endfunction
 
 if exists('+shellslash')
 
-  let s:dir_commit_file = '\c^fugitive://\%(/[^/]\@=\)\=\(.\{-1,\}\)//\%(\(\x\{40,\}\|[0-3]\)/\=\(/.*\)\=\)\=$'
+  let s:dir_commit_file = '\c^fugitive://\%(/[^/]\@=\)\=\(.\{-1,\}\)//\%(\(\x\{40,\}\|[0-3]\)\(/.*\)\=\)\=$'
 
   function! s:Slash(path) abort
     return tr(a:path, '\', '/')