The `ctx.Repo.RefName` was used to be a "short name", it causes a lot of
ambiguity.
This PR does some refactoring and use `RefFullName` to replace the
legacy `RefName`, and simplify RepoAssignment
Introduce RequestContext: is a short-lived context that is used to store
request-specific data.
RequestContext could be used to clean form tmp files, close context git
repo, and do some tracing in the future.
Then a lot of legacy code could be removed or improved. For example:
most `ctx.Repo.GitRepo.Close()` could be removed because the git repo
could be closed when the request is done.
