Memoize the source_branch_exists method for a MR

Each time this will make a redis call, which is expensive if called
enough times. This was an issue when serializing large diffs for a merge
request.

Clear the memoized source_branch_exists on MR update
Clear memoized variables before proceeding with checks

Issue: gitlab.com/gitlab-org/gitlab/-/issues/209786
Merge Request: gitlab.com/gitlab-org/gitlab/-/merge_requests/34516