tags: implement persistent tag caching (issue548). - rename findglobaltags() to findglobaltags1() (so the "no cache" implementation is still there if we need it) - add findglobaltags2() and make findglobaltags() an alias for it (disabling tag caching is a one-line patch) - factor out tagcache class with methods readcache() and writecache(); the expensive part of tag finding (iterate over heads and find .hgtags filenode) is now in tagcache.readcache()

localrepo: add destroyed() method for strip/rollback to use (issue548).

Factor tags module out of localrepo (issue548). Currently only handles reading tags, and will soon grow support for tag caching. Could eventually deal with updating tags too.

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