Merge branch 'jk/maint-http-half-auth-push' into maint-1.7.11

Merge branch 'jk/maint-http-half-auth-push' into maint-1.7.11

Pushing to smart HTTP server with recent Git fails without having
the username in the URL to force authentication, if the server is
configured to allow GET anonymously, while requiring authentication
for POST.

* jk/maint-http-half-auth-push:
  http: prompt for credentials on failed POST
  http: factor out http error code handling
  t: test http access to "half-auth" repositories
  t: test basic smart-http authentication
  t/lib-httpd: recognize */smart/* repos as smart-http
  t/lib-httpd: only route auth/dumb to dumb repos
  t5550: factor out http auth setup
  t5550: put auth-required repo in auth/dumb