cache: add a function to read an object ID from a buffer

In various places throughout the codebase, we need to read data into a
struct object_id from a pack or other unsigned char buffer.  Add an
inline function that does this based on the current hash algorithm in
use, and use it in several places.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -523,7 +523,7 @@ static struct cache_tree *read_one(const char **buffer, unsigned long *size_p)
 	if (0 <= it->entry_count) {
 		if (size < rawsz)
 			goto free_return;
-		memcpy(it->oid.hash, (const unsigned char*)buf, rawsz);
+		oidread(&it->oid, (const unsigned char *)buf);
 		buf += rawsz;
 		size -= rawsz;
 	}

cache.h

@@ -1008,6 +1008,11 @@ static inline void oidclr(struct object_id *oid)
 	memset(oid->hash, 0, GIT_MAX_RAWSZ);
 }
 
+static inline void oidread(struct object_id *oid, const unsigned char *hash)
+{
+	memcpy(oid->hash, hash, the_hash_algo->rawsz);
+}
+
 
 #define EMPTY_TREE_SHA1_HEX \
 	"4b825dc642cb6eb9a060e54bf8d69288fbee4904"

resolve-undo.c

@@ -90,7 +90,7 @@ struct string_list *resolve_undo_read(const char *data, unsigned long size)
 				continue;
 			if (size < rawsz)
 				goto error;
-			memcpy(ui->oid[i].hash, (const unsigned char *)data, rawsz);
+			oidread(&ui->oid[i], (const unsigned char *)data);
 			size -= rawsz;
 			data += rawsz;
 		}