pack-bitmap-write: use const for hashes

The next change will use a const array when calling this method. There
is no need for the non-const version, so let's do this cleanup quickly.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap-write.c

@@ -683,7 +683,7 @@ static void write_hash_cache(struct hashfile *f,
 	}
 }
 
-void bitmap_writer_set_checksum(unsigned char *sha1)
+void bitmap_writer_set_checksum(const unsigned char *sha1)
 {
 	hashcpy(writer.pack_checksum, sha1);
 }

pack-bitmap.h

@@ -75,7 +75,7 @@ int bitmap_has_oid_in_uninteresting(struct bitmap_index *, const struct object_i
 off_t get_disk_usage_from_bitmap(struct bitmap_index *, struct rev_info *);
 
 void bitmap_writer_show_progress(int show);
-void bitmap_writer_set_checksum(unsigned char *sha1);
+void bitmap_writer_set_checksum(const unsigned char *sha1);
 void bitmap_writer_build_type_index(struct packing_data *to_pack,
 				    struct pack_idx_entry **index,
 				    uint32_t index_nr);