tags: support 'instant' tag retrieval (issue548) - modify _readtagcache() and _writetagcache() to read/write tag->node mapping for global tags - if (and only if) tip unchanged, use that cached mapping to avoid reading any revisions of .hgtags - change so tag names are UTF-8 in memory in tags.py, and converted to local encoding as late as possible (in localrepository._findtags())