Bitbucket: don't fail on PRs without shource sha
Some old Bitbucket PRs can miss the source commit information, presumably because the source repo has been removed since then. Here's an example from PyPy:
log extract:
PR iid 323, error undefined method `fetch' for nil:NilClass, trace: /opt/gitlab/embedded/service/gitlab-rails/lib/bitbucket/representation/pull_request.rb:61:in `source_branch_sha'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/bitbucket_import/importer.rb:229:in `block in import_pull_requests'
``
raw data from Bitbucket excerpt:
"source"=>{"commit"=>nil, "repository"=>nil, "branch"=>{"name"=>"py3k"}}, "comment_count"=>0, "state"=>"MERGED",
Looks like a general conversion from several `fetch` calls to `dig` would be in order in `lib/bitbucket/representation`