[PATCH 0 of 8] Persistent tag caching (now with tags module)

[Greg Ward]

Update of the patch series I sent last night.  Major changes:

- factored out new module, mercurial.tags

Oh wait.  That's it, that's the major change.  Things I haven't done
yet:

- drop separate tagcache class (just use functions)
- cache .hgtags content

Just wanted to see how everyone feels about having a separate tags
module.  I like it: now I don't feel nearly as bad about adding 200
lines of code to localrepo.py, because instead I'm removing 100 lines
from it.  

Greg