[PATCH 2 of 8] localrepo: rename in-memory tag cache instance attributes

[timeless]

> +        # These two define the set of tags for this repository.  _tags
> +        # maps tag name to node;

my initial response was to say you wanted "nodes", but i think for
proper English it would be "maps a tag name to a node".

>  _tagtypes maps tag name to 'global' or 'local'.

and again "a tag name"

(there's an alternative of using "each" instead of "a", but it implies
that all tag names are in your maps, and i'm not sure if that's true.)