rework git check-attr interface

Now gitattributes are looked up, efficiently, in only the places that
really need them, using the same approach used for cat-file.

The old CheckAttr code seemed very fragile, in the way it streamed files
through git check-attr.
I actually found that cad8824852aa0623dc41eac02a9e2bae47d88ec4
was still deadlocking with ghc 7.4, at the end of adding a lot of files.
This should fix that problem, and avoid future ones.

The best part is that this removes withAttrFilesInGit and withNumCopies,
which were complicated Seek methods, as well as simplfying the types
for several other Seek methods that had a Backend tupled in.

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