Consider xdfdiag files as binary

Unlike other text files, .xdfdiag files are completely generated and maintained by
Orcc network editor. Although they contains only text (xml content), it is
difficult to read diffs and merge them. The line ending on these files also doesn't
matter (we usually don't edit them manually).

For these reasons, xdfdiag files can be considered as binary. If this config
cause any issue in the usual usage of this repository, please open an issue.

In the future, maybe a hook could be added to automatically delete conflicting
diagrams onmerge.