What are the best lightweight tools for collaboratively keeping track of requirements for software projects that support versioning?

GitKraken can be connected to Github, Gitlab, or Bitbucket accounts through OAuth. From that point onward most if not all actions that are related to these services can be done inside GitKraken. Things like: cloning or forking a repository, adding a remote, pushing to a remote repository hosted on these services can be done inside the app.

You can even manage pull requests inside GitKraken for example. All pull requests for a certain branch for example are shown on that branch's graph.