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 6e9af863 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, 6e9af863 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: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

pager.c

@@ -11,8 +11,6 @@
  * something different on Windows.
  */
 
-static int spawned_pager;
-
 #ifndef WIN32
 static void pager_preexec(void)
 {
@@ -78,7 +76,7 @@ void setup_pager(void)
 	if (!pager)
 		return;
 
-	spawned_pager = 1; /* means we are emitting to terminal */
+	setenv("GIT_PAGER_IN_USE", "true", 1);
 
 	/* spawn the pager */
 	pager_argv[0] = pager;
@@ -109,10 +107,6 @@ void setup_pager(void)
 int pager_in_use(void)
 {
 	const char *env;
-
-	if (spawned_pager)
-		return 1;
-
 	env = getenv("GIT_PAGER_IN_USE");
 	return env ? git_config_bool("GIT_PAGER_IN_USE", env) : 0;
 }

t/t7006-pager.sh

@@ -181,6 +181,17 @@ test_expect_success 'color when writing to a file intended for a pager' '
 	colorful colorful.log
 '
 
+test_expect_success TTY 'colors are sent to pager for external commands' '
+	test_config alias.externallog "!git log" &&
+	test_config color.ui auto &&
+	(
+		TERM=vt100 &&
+		export TERM &&
+		test_terminal git -p externallog
+	) &&
+	colorful paginated.out
+'
+
 # Use this helper to make it easy for the caller of your
 # terminal-using function to specify whether it should fail.
 # If you write