Provide the merge=union semantics we want through an info/attributes file

Provide the merge=union semantics we want through an info/attributes file

This is much cleaner than we were doing before, (where we requiring a
nogit repository to provide its own .giattribtues file with the
merge=union configuration).

This is also much cleaner than an approach we had been considering
which was to temporarily move away any .gitattributes file, replacing
it with our own for the time of "nogit sync", and then restoring it.

Here, we take advantage of the define git semantics that:

1. A clone does not bring down .git/info/attributes so we are
   guaranteed to be safe to provide our own content here.

2. The content we provide in .git/info/attributes overrides anything
   set in any .gitattributes files so we can be confident that our
   configuration here will apply in all cases.