pager: introduce wait_for_pager

Since f67b45f862 (Introduce trivial new pager.c helper infrastructure,
2006-02-28) we have the machinery to send our output to a pager.

That machinery, once set up, does not allow us to regain the original
stdio streams.

In the interactive commands (i.e.: add -p) we want to use the pager for
some output, while maintaining the interaction with the user.

Modify the pager machinery so that we can use `setup_pager()` and, once
we've finished sending the desired output for the pager, wait for the
pager termination using a new function `wait_for_pager()`.  Make this
function reset the pager machinery before returning.

One specific point to note is that we avoid forking the pager in
`setup_pager()` if the configured pager is an empty string [*1*] or
simply "cat" [*2*].  In these cases, `setup_pager()` does nothing and
therefore `wait_for_pager()` should not be called.

We could modify `setup_pager()` to return an indication of these
situations, so we could avoid calling `wait_for_pager()`.

However, let's avoid transferring that responsibility to the caller and
instead treat the call to `wait_for_pager()` as a no-op when we know we
haven't forked the pager.

   1.- 402461aab1 (pager: do not fork a pager if PAGER is set to empty.,
                   2006-04-16)

   2.- caef71a535 (Do not fork PAGER=cat, 2006-04-16)

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

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