tags: read tag info into a sorted dict (rather than into a regular dict) This lets us preserve the original tag order (as specified on the .hgtags file that is being read). This will be useful to preserve the tag order when saving the result of a successful automated .hgtags merge (which will be introduced on a future patch). There shouldn't be much impact on performance because the sortdict that _readtags returns is then used to update the alltags regular dict (which remains a regular dict).