.gitattributes: must not add the more generic rule at the end

[yarikoptic]

ATM we go through entries and add if they aren't already there in https://github.com/nipy/heudiconv/blob/HEAD/heudiconv/external/dlad.py#L73

It is quite wrong since the order of rules in .gitattributes matter and in my case it was managing to add that * annex.largefiles=(largerthan=100kb) to be the last one thus overloading the rules e.g for _scans.tsv.