pager: do allow spawning pager recursively

This reverts commit 88e8f90, which
tried to allow

    GIT_PAGER="git -p column --mode='dense color'" git -p branch

and still wanted to avoid "git -p column" to invoke itself.  However,
this falls into "don't do that -p then" category.

In particular, inside "git log", with results going through less, a
potentially interesting commit may be found and from there inside
"less", the user may want to execute "git show <commit>".  Before
the commit being reverted, this used to show the patch in less but
it no longer does.

Signed-off-by: Jörn Engel <joern@logfs.org>
Reviewed-by: Jeff King <peff@peff.net>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Acked-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pager.c

@@ -64,7 +64,7 @@ void setup_pager(void)
 {
 	const char *pager = git_pager(isatty(1));
 
-	if (!pager || pager_in_use())
+	if (!pager)
 		return;
 
 	/*