builtin.h: remove unneccessary includes

This also made it clear that a few .c files under builtin/ were
depending upon some headers but had forgotten to #include them.  Add the
missing direct includes while at it.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin.h

@@ -2,8 +2,6 @@
 #define BUILTIN_H
 
 #include "git-compat-util.h"
-#include "strbuf.h"
-#include "commit.h"
 
 /*
  * builtin API