object_array: use object_array_clear(), not free()

Instead of freeing `foo.objects` for an object array `foo` (sometimes
conditionally), call `object_array_clear(&foo)`. This means we don't
poke as much into the implementation, which is already a good thing, but
also that we release the individual entries as well, thereby fixing at
least one memory-leak (in diff-lib.c).

If someone is holding on to a pointer to an element's `name` or `path`,
that is now a dangling pointer, i.e., we'd be turning an unpleasant
situation into an outright bug. To the best of my understanding no such
long-term pointers are being taken.

The way we handle `study` in builting/reflog.c still looks like it might
leak. That will be addressed in the next commit.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

upload-pack.c

@@ -888,7 +888,7 @@ static void receive_needs(void)
 		}
 
 	shallow_nr += shallows.nr;
-	free(shallows.objects);
+	object_array_clear(&shallows);
 }
 
 /* return non-zero if the ref is hidden, otherwise 0 */