Prefer browsing upstream when on the master branch

When a user would do `hub browse` on the master branch and they had a
fork, the hub would open the browser with their fork instead of the
upstream project. This is surprising and unwanted, so I've added an
exception for this case.

Fixes #474
Go canonical when browsing to subpage other than commits/tree/blob/settings

If the user is currently on the "feature" branch that is pushed to their
fork, when they do `hub browse -- issues` it should open issues of the
main (canonical) repository, not that of the fork.
Merge branch 'browse-upstream'