Bug: Edge case in model selection leads to TypeError

[cdboer]

The model implemented in gitlab2prov.prov.model.modification assumes that all FileRevisions passed to it have a previous revision value that is not None. This is not always the case. The easiest example would be the first revision of every given file. As it marks the first revision, it can not have a previous one. The first revision of a file is captured by the gitlab2prov.prov.model.addition model, which is why this seemed like a non-issue during implementation.

We have chosen to select the modification model for other File.change_type values in addition to MODIFIED. The modification model is therefore applied to the following change statuses: MODIFIED, RENAMED, COPIED, CHANGED. Some of these evidently do not require a previous revision to exist. Which is why some revisions end up with no predecessor or a None value (compare).