pack-bitmap: introduce bitmap_writer_free()

Now that there is clearer memory ownership around the bitmap_writer
structure, introduce a bitmap_writer_free() function that callers may
use to free any memory associated with their instance of the
bitmap_writer structure.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap.h

@@ -139,6 +139,7 @@ void bitmap_writer_finish(struct bitmap_writer *writer,
 			  uint32_t index_nr,
 			  const char *filename,
 			  uint16_t options);
+void bitmap_writer_free(struct bitmap_writer *writer);
 char *midx_bitmap_filename(struct multi_pack_index *midx);
 char *pack_bitmap_filename(struct packed_git *p);