Use byte literals to make go vet happy.

The go vet command complains about untagged struct initializers when
defining a ShaHash directly.  This seems to be a limitation where go vet
does not exclude the warning for types which are a constant size byte
array like it does for normal constant size byte array definition.

This commit simply modifies the code to use a constant definition cast to
a ShaHash to overcome the limitation of go vet.