git-repo: open repository homepage (#140)

@kevgo @allewun 
You can use the multi-line form:

```ruby
case domain
when 'Bitbucket'
  sha = recent_commit_shas(1).join('')[0, 12]
  "https://bitbucket.org/Originate/git-town/pull-request/new?source=Originate%2Fgit-town%3A#{sha}%3A#{branch_name}"
when 'GitHub'
  "https://github.com/Originate/git-town/compare/#{branch_name}?expand=1"
else
  fail "Unknown domain: #{domain}"
end
```