pager: do not close fd 2 unnecessarily

pager: do not close fd 2 unnecessarily

We send errors to the pager since 61b80509e3 (sending errors to stdout
under $PAGER, 2008-02-16).

In a8335024c2 (pager: do not dup2 stderr if it is already redirected,
2008-12-15) an exception was introduced to avoid redirecting stderr if
it is not connected to a terminal.

In such exceptional cases, the close(STDERR_FILENO) we're doing in
close_pager_fds, is unnecessary.

Furthermore, in a subsequent commit we're going to introduce changes
that will involve using close_pager_fds multiple times.

With this in mind, controlling when we want to close stderr, become
sensible.

Let's close(STDERR_FILENO) only when necessary, and pave the way for the
upcoming changes.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>