[PATCH] pager: do not leak "GIT_PAGER_IN_USE" to the pager

Since 2e6c01e2, we export GIT_PAGER_IN_USE so that a process that
becomes the upstream of the spawned pager can still tell that we
have spawned the pager and decide to do colored output even when
its output no longer goes to a terminal (i.e. isatty(1)).

But we forgot to clear it from the enviornment of the spawned pager.
This is not a problem in a sane world, but if you have a handful of
thousands Git users in your organization, somebody is bound to do
strange things, e.g. typing "!<ENTER>" instead of 'q' to get control
back from $LESS.  GIT_PAGER_IN_USE is still set in that subshell
spawned by "less", and all sorts of interesting things starts
happening, e.g. "git diff | cat" starts coloring its output.

We can clear the environment variable in the half of the fork that
runs the pager to avoid the confusion.

Signed-off-by: Junio C Hamano <redacted>
---

 * Arguably, "git diff | cat" that is colored is much minor problem
   than that the user keep using that subshell from pager without
   realizing.  The user may run more git commands that spawn a pager
   and do the "!<ENTER>" infinite times creating a deep process tree
   and then "logout" many number of times.

 pager.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/pager.c b/pager.c
index 98b2682..070dc11 100644
--- a/pager.c
+++ b/pager.c

@@ -78,6 +78,7 @@ void setup_pager(void)
 		argv_array_push(&pager_process.env_array, "LESS=FRX");
 	if (!getenv("LV"))
 		argv_array_push(&pager_process.env_array, "LV=-c");
+	argv_array_push(&pager_process.env_array, "GIT_PAGER_IN_USE");
 	if (start_command(&pager_process))
 		return;