http: pass user to readauthforuri() (fix 4a43e23b8c55)
urllib2 never handles URIs with credentials, we have to extract them and store
them in the password manager before handing the stripped URI. Half of the
changes deducing the username from the URI in 4a43e23b8c55 were incorrect.
Instead, we retrieve the username from the password manager before passing to
readauthforuri().
test-hgweb-auth.py was passing because the test itself was flawed: it was
passing URIs with credentials to find_password(), which never happens.