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>

builtin-unpack-objects.c

@@ -8,8 +8,6 @@
 #include "tag.h"
 #include "tree.h"
 
-#include <sys/time.h>
-
 static int dry_run, quiet, recover, has_errors;
 static const char unpack_usage[] = "git-unpack-objects [-n] [-q] [-r] < pack-file";