Track base branch

[Sakib25800]

Prerequisite to #219

[Kobzol]

Thank you! The core of the implementation looks solid, but I have a couple of notes/questions.

[Sakib25800]

Squash and rebase?

[Kobzol]

Passing the base branch to the PR approve/unapprove/... methods is quite annoying. I still think that it is worth it to have the base branch non-nullable though, because a PR without a base branch doesn't really make any sense.

Could you please modify the DB methods that update PRs (e.g. approve/unapprove) to receive a PullRequestModel as a parameter, instead of PR number, base branch etc.? This will both ensure that a PR that we're trying to edit exists in the DB, and also it will avoid us having to pass N parameters to all these methods once we start adding more non-NULL attributes to PRs.

[Kobzol]

Btw before rebasing I would advise you to squash the changes, yes :) It will make the rebase much more pleasant.

[Sakib25800]

Fixed.

Looks much cleaner (using PullRequestModel as a param).

[Kobzol]

Thanks!