Include file cleanups..

Add <limits.h> to the include files handled by "cache.h", and remove
extraneous #include directives from various .c files. The rule is that
"cache.h" gets all the basic stuff, so that we'll have as few system
dependencies as possible.

commit.c

@@ -1,8 +1,6 @@
 #include "tag.h"
 #include "commit.h"
 #include "cache.h"
-#include <string.h>
-#include <limits.h>
 
 const char *commit_type = "commit";