Hide /net_skeleton.[ch] from git diff

Hide amalgamated files by treating them as binary files.
The users will still see that there are diffs but they won't be distracted
by duplicated diffs in every `git diff` output.


I got the idea from:
http://blog.andrewray.me/dealing-with-compiled-files-in-git/

It's a first step towards improving the handling of amalgamated 
and other kinds of generated files.