csum-file.c: extract algop from hashfile_checksum_valid()

Perform a similar transformation as in the previous commit, but focused
instead on hashfile_checksum_valid(). This function does not work with a
hashfile structure itself, and instead validates the raw contents of a
file written using the hashfile API.

We'll want to be prepared for a similar change to this function in the
future, so prepare ourselves for that by extracting 'the_hash_algo' into
its own field for use within this function.

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

Author: ttaylorr

csum-file.c

@@ -242,14 +242,15 @@ int hashfile_checksum_valid(const unsigned char *data, size_t total_len)
 {
 	unsigned char got[GIT_MAX_RAWSZ];
 	git_hash_ctx ctx;
-	size_t data_len = total_len - the_hash_algo->rawsz;
+	const struct git_hash_algo *algop = the_hash_algo;
+	size_t data_len = total_len - algop->rawsz;
 
-	if (total_len < the_hash_algo->rawsz)
+	if (total_len < algop->rawsz)
 		return 0; /* say "too short"? */
 
-	the_hash_algo->unsafe_init_fn(&ctx);
-	the_hash_algo->unsafe_update_fn(&ctx, data, data_len);
-	the_hash_algo->unsafe_final_fn(got, &ctx);
+	algop->unsafe_init_fn(&ctx);
+	algop->unsafe_update_fn(&ctx, data, data_len);
+	algop->unsafe_final_fn(got, &ctx);
 
-	return hasheq(got, data + data_len, the_repository->hash_algo);
+	return hasheq(got, data + data_len, algop);
 }