control, hookd, web, wiki: the default branch can be set, and follows the first push !337

post-receive moves an unborn HEAD to the first branch a push creates and records it, so a first push of master or trunk clones with a checkout. repo settings default-branch <owner/name> <branch> moves it later and refuses a branch that does not exist; the settings page carries the same control. Parity row and a Users line. e2e covers the first push, the setting, the refusals, and the web field.

Closes #189