Simplify diffing & merging of message & parser generated files through .gitattributes

There was some interest in this on mattermost so it's worth creating an issue.

Generated messages & generated parser files are meaningless to inspect. Their source files are easier to read. So there are two things we can do with git attributes.:

• Not clutter entire screens with differences of generated files when diff-ing branches or commits or so on. This can be done via the diff=nodiff attribute.
• Temporarily trick git into solving these spurious conflicts on generated files since a regeneration has to happen anyway. This can be done via merge=ours or merge=theirs attributes. Almost certainly both are wrong, but there's no magical merge=$(make messages && make parsers) so they would have to do. Advantage of merge=ours: zero diff, but you will probably have other conflicts and that means other modified files so it let's you focus on them. Advantage of merge=theirs: some diff which will tell you nothing more than what files need regenerating.

Git attributes are assigned per file or directory. Since we have generated message & parser files in open directories with other source code files, our attributes would work directly associated with the generated files.