simplify inclusion of system header files.

This is a mechanical clean-up of the way *.c files include
system header files.

 (1) sources under compat/, platform sha-1 implementations, and
     xdelta code are exempt from the following rules;

 (2) the first #include must be "git-compat-util.h" or one of
     our own header file that includes it first (e.g. config.h,
     builtin.h, pkt-line.h);

 (3) system headers that are included in "git-compat-util.h"
     need not be included in individual C source files.

 (4) "git-compat-util.h" does not have to include subsystem
     specific header files (e.g. expat.h).

Signed-off-by: Junio C Hamano <junkio@cox.net>

git.c

@@ -1,20 +1,8 @@
-#include <stdio.h>
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <dirent.h>
-#include <unistd.h>
-#include <stdlib.h>
-#include <string.h>
-#include <errno.h>
-#include <limits.h>
-#include <stdarg.h>
-#include "git-compat-util.h"
+#include "builtin.h"
 #include "exec_cmd.h"
 #include "cache.h"
 #include "quote.h"
 
-#include "builtin.h"
-
 const char git_usage_string[] =
 	"git [--version] [--exec-path[=GIT_EXEC_PATH]] [-p|--paginate] [--bare] [--git-dir=GIT_DIR] [--help] COMMAND [ARGS]";