Support GIT_PAGER_IN_USE environment variable

When deciding whether or not to turn on automatic color
support, git_config_colorbool checks whether stdout is a
tty. However, because we run a pager, if stdout is not a
tty, we must check whether it is because we started the
pager. This used to be done by checking the pager_in_use
variable.

This variable was set only when the git program being run
started the pager; there was no way for an external program
running git indicate that it had already started a pager.
This patch allows a program to set GIT_PAGER_IN_USE to a
true value to indicate that even though stdout is not a tty,
it is because a pager is being used.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -608,7 +608,7 @@ extern int write_or_whine_pipe(int fd, const void *buf, size_t count, const char
 /* pager.c */
 extern void setup_pager(void);
 extern char *pager_program;
-extern int pager_in_use;
+extern int pager_in_use(void);
 extern int pager_use_color;
 
 extern char *editor_program;

color.c

@@ -135,7 +135,7 @@ int git_config_colorbool(const char *var, const char *value, int stdout_is_tty)
  auto_color:
 	if (stdout_is_tty < 0)
 		stdout_is_tty = isatty(1);
-	if (stdout_is_tty || (pager_in_use && pager_use_color)) {
+	if (stdout_is_tty || (pager_in_use() && pager_use_color)) {
 		char *term = getenv("TERM");
 		if (term && strcmp(term, "dumb"))
 			return 1;

environment.c

@@ -31,7 +31,6 @@ size_t packed_git_window_size = DEFAULT_PACKED_GIT_WINDOW_SIZE;
 size_t packed_git_limit = DEFAULT_PACKED_GIT_LIMIT;
 size_t delta_base_cache_limit = 16 * 1024 * 1024;
 char *pager_program;
-int pager_in_use;
 int pager_use_color = 1;
 char *editor_program;
 char *excludes_file;

pager.c

@@ -5,6 +5,8 @@
  * something different on Windows, for example.
  */
 
+static int spawned_pager;
+
 static void run_pager(const char *pager)
 {
 	/*
@@ -41,7 +43,7 @@ void setup_pager(void)
 	else if (!*pager || !strcmp(pager, "cat"))
 		return;
 
-	pager_in_use = 1; /* means we are emitting to terminal */
+	spawned_pager = 1; /* means we are emitting to terminal */
 
 	if (pipe(fd) < 0)
 		return;
@@ -70,3 +72,14 @@ void setup_pager(void)
 	die("unable to execute pager '%s'", pager);
 	exit(255);
 }
+
+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;
+}