use SWAP macro

Apply the semantic patch swap.cocci to convert hand-rolled swaps to use
the macro SWAP.  The resulting code is shorter and easier to read, the
object code is effectively unchanged.

The patch for object.c had to be hand-edited in order to preserve the
comment before the change; Coccinelle tried to eat it for some reason.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -109,9 +109,7 @@ extern void strbuf_attach(struct strbuf *, void *, size_t, size_t);
  */
 static inline void strbuf_swap(struct strbuf *a, struct strbuf *b)
 {
-	struct strbuf tmp = *a;
-	*a = *b;
-	*b = tmp;
+	SWAP(*a, *b);
 }