fix(cli): when we generate a file that exists and has the same content, skip writing

[W95Psp]

This PR:

• prints relative paths instead of absolute ones in hax note messages
• write files only when a file doesn't exists or has a different content as what we were about to write
• prints the notice message hax: unchanged file <path> instead of hax: wrote file <path> in those cases

This is useful not to invalidate checked files in caches while typechecking F*