sha1_file: convert hash_sha1_file to object_id

Convert the declaration and definition of hash_sha1_file to use
struct object_id and adjust all function calls.

Rename this function to hash_object_file.

Signed-off-by: Patryk Obara <patryk.obara@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diffcore-rename.c b/diffcore-rename.c
index 245e999..e27d0bb 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -260,8 +260,8 @@ static unsigned int hash_filespec(struct diff_filespec *filespec)
 	if (!filespec->oid_valid) {
 		if (diff_populate_filespec(filespec, 0))
 			return 0;
-		hash_sha1_file(filespec->data, filespec->size, "blob",
-			       filespec->oid.hash);
+		hash_object_file(filespec->data, filespec->size, "blob",
+				 &filespec->oid);
 	}
 	return sha1hash(filespec->oid.hash);
 }