ReplaceCmd: redirect stderr to tmp file / buffer

The option `status.showUntrackedFiles=all` used with `git status` for
`:Gstatus` might cause an error, which then causes fugitive to display an
empty status window / index file.

Redirecting the stderr output is useful in this case.

The generated command was:

    git --git-dir=/home/user/.dotfiles/.git -c 'status.displayCommentPrefix=true' -c 'color.status=false' -c 'status.short=false' -c 'status.showUntrackedFiles=all' status

The error from git is related to submodules being moved to another
subdirectory, where the relative "gitdir" now does not exist anymore:

    fatal: Not a git repository: vim/bundle.old.nobackup/CLEAN/colorscheme-base16/../../../.git/modules/vim/bundle/colorscheme-base16

While that's a Git / user error after all, fugitive should be more
helpful in that case by displaying the error.

It uses the 'shellpipe' setting to detect if '2>&1' is supported (Ref:
https://github.com/tpope/vim-fugitive/pull/661#issuecomment-120438667).

Closes #661.

plugin/fugitive.vim

@@ -2398,11 +2398,15 @@ function! s:ReplaceCmd(cmd,...) abort
         let prefix = 'env GIT_INDEX_FILE='.s:shellesc(a:1).' '
       endif
     endif
+    let redir = ' > '.tmp
+    if &shellpipe =~ '2>&1'
+      let redir .= ' 2>&1'
+    endif
     if s:winshell()
       let cmd_escape_char = &shellxquote == '(' ?  '^' : '^^^'
-      call system('cmd /c "'.prefix.s:gsub(a:cmd,'[<>]', cmd_escape_char.'&').' > '.tmp.'"')
+      call system('cmd /c "'.prefix.s:gsub(a:cmd,'[<>]', cmd_escape_char.'&').redir.'"')
     else
-      call system(' ('.prefix.a:cmd.' > '.tmp.') ')
+      call system(' ('.prefix.a:cmd.redir.') ')
     endif
   finally
     if exists('old_index')