pager: do not dup2 stderr if it is already redirected
An earlier commit 61b8050 (sending errors to stdout under $PAGER, 2008-02-16) avoided losing the error messages that are sent to the standard error when $PAGER is in effect by dup2'ing fd 2 to the pager. his way, showing a tag object that points to a bad object:
$ git show tag-foo
would give the error message to the pager. However, it was not quite right if the user did:
$ git show 2>error.log tag-foo
i.e. use the pager but store the errors in a separate file.