hgwebdir: read --webdir-conf as actual configuration to ui (issue1586)
This cleans up code and allows specification of values more globally. For
example, it's now possible to specify web.contact in webdir-conf for all
repositories without a specified contact set.
http://example.com/no/anchor, revs: None, checkout: None
http://example.com/an/anchor, revs: ['foo'], checkout: 'foo'
http://example.com/no/anchor/revs, revs: ['foo'], checkout: 'foo'
http://example.com/an/anchor/revs, revs: ['foo', 'bar'], checkout: 'foo'
http://example.com/an/anchor/rev-None, revs: ['foo'], checkout: 'foo'