API: Fixes return code when accessing non existent branch (#2922)

Merge branch 'easy-to-find-commit-on-network-graph' of https://github.com/hiroponz/gitlabhq into hiroponz-easy-to-find-commit-on-network-graph

Disable autocomplete for admin/users form

Gitlab incorrectly routes blobs to the compare controller depending on filename.

Merge branch 'fix-db-after-updating-rails' of https://github.com/Undev/gitlabhq into Undev-fix-db-after-updating-rails

Instead of using funtion `error!` the function `not_found!` is used to return 404 error. Adjusted documentation accordingly.

Accessing a repository branch that does not exist returns a 404 error instead
of 200 now. Added a test.