Fix handling of paths when run outside the repo. The main problem was that dirstate.getcwd() returned just "", which was interpreted as "we're at the repo root". It now returns an absolute path. The util.pathto function was also changed to deal with the "cwd is an absolute path" case.

pass repo.root to util.pathto() in preparation for the next patch

hgweb: handle IOErrors and OSErrors during unbundle This allows the client to display a reasonable message to the user (e.g. "Permission denied: .hg/lock"), instead of the current "<url> does not appear to be an hg repository".

hgweb.unbundle: call req.httphdr only after the last possible call to bail