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()