spawn pager via run_command interface

This has two important effects:

 1. The pager is now the _child_ process, instead of the
    parent. This means that whatever spawned git (e.g., the
    shell) will see the exit code of the git process, and
    not the pager.

 2. The mingw and regular code are now unified, which makes
    the setup_pager function much simpler.

There are two caveats:

 1. We used to call execlp directly on the pager, followed
    by trying to exec it via the shall. We now just use the
    shell (which is what mingw has always done). This may
    have different results for pager names which contain
    shell metacharacters.

    It is also slightly less efficient because we
    unnecessarily run the shell; however, pager spawning is
    by definition an interactive task, so it shouldn't be
    a huge problem.

 2. The git process will remain in memory while the user
    looks through the pager. This is potentially wasteful.
    We could get around this by turning the parent into a
    meta-process which spawns _both_ git and the pager,
    collects the exit status from git, waits for both to
    end, and then exits with git's exit code.

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

pager.c

@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "run-command.h"
 
 /*
  * This is split up from the rest of git so that we can do
@@ -8,7 +9,7 @@
 static int spawned_pager;
 
 #ifndef __MINGW32__
-static void run_pager(const char *pager)
+static void pager_preexec(void)
 {
 	/*
 	 * Work around bug in "less" by not starting it until we
@@ -20,17 +21,13 @@ static void run_pager(const char *pager)
 	FD_SET(0, &in);
 	select(1, &in, NULL, &in, NULL);
 
-	execlp(pager, pager, NULL);
-	execl("/bin/sh", "sh", "-c", pager, NULL);
+	setenv("LESS", "FRSX", 0);
 }
-#else
-#include "run-command.h"
+#endif
 
 static const char *pager_argv[] = { "sh", "-c", NULL, NULL };
-static struct child_process pager_process = {
-	.argv = pager_argv,
-	.in = -1
-};
+static struct child_process pager_process;
+
 static void wait_for_pager(void)
 {
 	fflush(stdout);
@@ -40,14 +37,9 @@ static void wait_for_pager(void)
 	close(2);
 	finish_command(&pager_process);
 }
-#endif
 
 void setup_pager(void)
 {
-#ifndef __MINGW32__
-	pid_t pid;
-	int fd[2];
-#endif
 	const char *pager = getenv("GIT_PAGER");
 
 	if (!isatty(1))
@@ -66,37 +58,13 @@ void setup_pager(void)
 
 	spawned_pager = 1; /* means we are emitting to terminal */
 
-#ifndef __MINGW32__
-	if (pipe(fd) < 0)
-		return;
-	pid = fork();
-	if (pid < 0) {
-		close(fd[0]);
-		close(fd[1]);
-		return;
-	}
-
-	/* return in the child */
-	if (!pid) {
-		dup2(fd[1], 1);
-		dup2(fd[1], 2);
-		close(fd[0]);
-		close(fd[1]);
-		return;
-	}
-
-	/* The original process turns into the PAGER */
-	dup2(fd[0], 0);
-	close(fd[0]);
-	close(fd[1]);
-
-	setenv("LESS", "FRSX", 0);
-	run_pager(pager);
-	die("unable to execute pager '%s'", pager);
-	exit(255);
-#else
 	/* spawn the pager */
 	pager_argv[2] = pager;
+	pager_process.argv = pager_argv;
+	pager_process.in = -1;
+#ifndef __MINGW32__
+	pager_process.preexec_cb = pager_preexec;
+#endif
 	if (start_command(&pager_process))
 		return;
 
@@ -107,7 +75,6 @@ void setup_pager(void)
 
 	/* this makes sure that the parent terminates after the pager */
 	atexit(wait_for_pager);
-#endif
 }
 
 int pager_in_use(void)