localrepo: factor updatetags() out of readtags() (issue548).

localrepo: improve readability of _findtags(), readtags() (issue548). - rename many local variables - add some comments - refactor call to line.split() (catch ValueError rather than checking length of return value: one less local variable)

localrepo: rename in-memory tag cache instance attributes (issue548). - self.tagscache to self._tags - self._tagstypecache to self._tagtypes - this is for consistency, readability, privacy, and to subtly hint that "caching" is something else

localrepo: factor _findtags() out of tags() (issue548). This makes in-memory caching the sole responsibility of localrepo, eliminating some localrepo code that was duplicated in mq and bookmarks.