object-file API: have write_object_file() take "enum object_type"

Change the write_object_file() function to take an "enum object_type"
instead of a "const char *type". Its callers either passed
{commit,tree,blob,tag}_type and can pass the corresponding OBJ_* type
instead, or were hardcoding strings like "blob".

This avoids the back & forth fragility where the callers of
write_object_file() would have the enum type, and convert it
themselves via type_name(). We do have to now do that conversion
ourselves before calling write_object_file_prepare(), but those
codepaths will be similarly adjusted in subsequent commits.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

match-trees.c

@@ -235,7 +235,7 @@ static int splice_tree(const struct object_id *oid1, const char *prefix,
 		rewrite_with = oid2;
 	}
 	hashcpy(rewrite_here, rewrite_with->hash);
-	status = write_object_file(buf, sz, tree_type, result);
+	status = write_object_file(buf, sz, OBJ_TREE, result);
 	free(buf);
 	return status;
 }