sha1_file: convert write_sha1_file to object_id

Convert the definition and declaration of write_sha1_file to
struct object_id and adjust usage of this function.

This commit also converts static function write_sha1_file_prepare, as it
is closely related.

Rename these functions to write_object_file and
write_object_file_prepare respectively.

Replace sha1_to_hex, hashcpy and hashclr with their oid equivalents
wherever possible.

Signed-off-by: Patryk Obara <patryk.obara@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -1567,7 +1567,7 @@ int commit_tree_extended(const char *msg, size_t msg_len,
 		goto out;
 	}
 
-	result = write_sha1_file(buffer.buf, buffer.len, commit_type, ret->hash);
+	result = write_object_file(buffer.buf, buffer.len, commit_type, ret);
 out:
 	strbuf_release(&buffer);
 	return result;