Diff: metadata file part of patch

[ben-edna]

When generating a patch using dfetch diff, the metadata file (.dfetch_data.yaml) can be part of the patch:

[spoorcc]

This leads to unexpected behavior:

Given a project is fetched and checked in
And it has local changes
When a patch is created with dfetch diff
Then the metadata is part of patch
When this patch is added to the project entry in the manifest
Then dfetch update ignores the patch