Teach git var about GIT_PAGER

Expose the command found by setup_pager() for scripts to use.
Scripts can use this to avoid repeating the logic to look for a
proper pager in each command.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-var.txt

@@ -44,6 +44,12 @@ GIT_EDITOR::
     environment variable, then `core.editor` configuration, then
     `$VISUAL`, then `$EDITOR`, and then finally 'vi'.
 
+GIT_PAGER::
+    Text viewer for use by git commands (e.g., 'less').  The value
+    is meant to be interpreted by the shell.  The order of preference
+    is the `$GIT_PAGER` environment variable, then `core.pager`
+    configuration, then `$PAGER`, and then finally 'less'.
+
 Diagnostics
 -----------
 You don't exist. Go away!::

cache.h

@@ -751,6 +751,7 @@ extern const char *git_committer_info(int);
 extern const char *fmt_ident(const char *name, const char *email, const char *date_str, int);
 extern const char *fmt_name(const char *name, const char *email);
 extern const char *git_editor(void);
+extern const char *git_pager(void);
 
 struct checkout {
 	const char *base_dir;

pager.c

@@ -44,12 +44,14 @@ static void wait_for_pager_signal(int signo)
 	raise(signo);
 }
 
-void setup_pager(void)
+const char *git_pager(void)
 {
-	const char *pager = getenv("GIT_PAGER");
+	const char *pager;
 
 	if (!isatty(1))
-		return;
+		return NULL;
+
+	pager = getenv("GIT_PAGER");
 	if (!pager) {
 		if (!pager_program)
 			git_config(git_default_config, NULL);
@@ -60,6 +62,16 @@ void setup_pager(void)
 	if (!pager)
 		pager = "less";
 	else if (!*pager || !strcmp(pager, "cat"))
+		pager = NULL;
+
+	return pager;
+}
+
+void setup_pager(void)
+{
+	const char *pager = git_pager();
+
+	if (!pager)
 		return;
 
 	spawned_pager = 1; /* means we are emitting to terminal */

var.c

@@ -18,6 +18,15 @@ static const char *editor(int flag)
 	return pgm;
 }
 
+static const char *pager(int flag)
+{
+	const char *pgm = git_pager();
+
+	if (!pgm)
+		pgm = "cat";
+	return pgm;
+}
+
 struct git_var {
 	const char *name;
 	const char *(*read)(int);
@@ -26,6 +35,7 @@ static struct git_var git_vars[] = {
 	{ "GIT_COMMITTER_IDENT", git_committer_info },
 	{ "GIT_AUTHOR_IDENT",   git_author_info },
 	{ "GIT_EDITOR", editor },
+	{ "GIT_PAGER", pager },
 	{ "", NULL },
 };