ignore global core.attributesFile

[andreineculau]

scenario: you have a global .gitattributes for settings crlf, text/binary, diff etc settings

then this is what you get

$ transcrypt --list-contexts                                                                                                                                                             default (no patterns in .gitattributes)
custom (no patterns in .gitattributes)

instead of

$ transcrypt --list-contexts                                                                                                                                                             default
custom

although the .gitattributes in the repo has the patterns defined

Point is: do we really support transcrypt globally? with custom contexts?
If we do, then we need to check all attributes files, not only the first one.

PS: on a different note, it also doesn't make sense (to me at least) that --add=<pattern> write to anything but a local gitattributes

[jmurty]

The check against non-local .gitattributes was added in #137 and although I don't tend to us config files like .gitattributes from outside the immediate repo directory, some people do. I can imagine for example situations where you want transcrypt to encrypt *.secret files across all your projects, in which case putting this in a global .gitattributes would make sense.

So I think the right fix here would be to check all attributes files, which won't necessarily be easy.

[jmurty]

See a potential alternative solution that finds contexts across all gitattributes files, not just one, in #202