pager: environment variable GIT_PAGER to override PAGER

Signed-off-by: Matthias Lederhofer <matled@gmx.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git.txt

@@ -627,6 +627,9 @@ git Diffs
 
 other
 ~~~~~
+'GIT_PAGER'::
+	This environment variable overrides `$PAGER`.
+
 'GIT_TRACE'::
 	If this variable is set git will print `trace:` messages on
 	stderr telling about alias expansion, built-in command

pager.c

@@ -15,10 +15,12 @@ void setup_pager(void)
 {
 	pid_t pid;
 	int fd[2];
-	const char *pager = getenv("PAGER");
+	const char *pager = getenv("GIT_PAGER");
 
 	if (!isatty(1))
 		return;
+	if (!pager)
+		pager = getenv("PAGER");
 	if (!pager)
 		pager = "less";
 	else if (!*pager || !strcmp(pager, "cat"))