setup_pager: set GIT_PAGER_IN_USE

setup_pager: set GIT_PAGER_IN_USE

We have always set a global "spawned_pager" variable when we
start the pager. This lets us make the auto-color decision
later in the program as as "we are outputting to a terminal,
or to a pager which can handle colors".

Commit 6e9af86 added support for the GIT_PAGER_IN_USE
environment variable. An external program calling git (e.g.,
git-svn) could set this variable to indicate that it had
already started the pager, and that the decision about
auto-coloring should take that into account.

However, 6e9af86 failed to do the reverse, which is to tell
external programs when git itself has started the pager.
Thus a git command implemented as an external script that
has the pager turned on (e.g., "git -p stash show") would
not realize it was going to a pager, and would suppress
colors.

This patch remedies that; we always set GIT_PAGER_IN_USE
when we start the pager, and the value is respected by both
this program and any spawned children.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>