wt-status: introduce wt_status_state_free_buffers()

When we have a `struct wt_status_state`, we manually free its `branch`,
`onto` and `detached_from`, or sometimes just one or two of them.
Provide a function `wt_status_state_free_buffers()` which does the
freeing.

The callers are still aware of these fields, e.g., they check whether
`branch` was populated or not. But this way, they don't need to know
about *all* of them, and if `struct wt_status_state` gets more fields,
they will not need to learn to free them.

Users of `struct wt_status` (which contains a `wt_status_state`) already
have `wt_status_collect_free_buffers()` (corresponding to
`wt_status_collect()`) which we can also teach to use this new helper.

Finally, note that we're currently leaving dangling pointers behind.
Some callers work on a stack-allocated struct, where this is obviously
ok. But for the users of `run_status()` in builtin/commit.c, there are
ample opportunities for someone to mistakenly use those dangling
pointers. We seem to be ok for now, but it's a use-after-free waiting to
happen. Let's leave NULL-pointers behind instead.

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

wt-status.c

@@ -778,9 +778,14 @@ void wt_status_collect(struct wt_status *s)
 
 void wt_status_collect_free_buffers(struct wt_status *s)
 {
-	free(s->state.branch);
-	free(s->state.onto);
-	free(s->state.detached_from);
+	wt_status_state_free_buffers(&s->state);
+}
+
+void wt_status_state_free_buffers(struct wt_status_state *state)
+{
+	FREE_AND_NULL(state->branch);
+	FREE_AND_NULL(state->onto);
+	FREE_AND_NULL(state->detached_from);
 }
 
 static void wt_longstatus_print_unmerged(struct wt_status *s)