This PR adds a new dropdown to select a commit or a commit range (shift-click like github) of a Pull Request. After selection of a commit only the changes of this commit will be shown. When selecting a range of commits the diff of this range is shown. This allows to review a PR commit by commit or by viewing only commit ranges. The "Show changes since your last review" mechanism github uses is implemented, too. When reviewing a single commit or a commit range the "Viewed" functionality is disabled. ## Screenshots ### The commit dropdown  ### Selecting a commit range  ### Show changes of a single commit only  ### Show changes of a commit range  Fixes https://github.com/go-gitea/gitea/issues/20989 Fixes https://github.com/go-gitea/gitea/issues/19263 --------- Co-authored-by: silverwind <me@silverwind.io> Co-authored-by: KN4CK3R <admin@oldschoolhack.me> Co-authored-by: wxiaoguang <wxiaoguang@gmail.com> Co-authored-by: delvh <dev.lh@web.de>
