Validate only if interesting files are changed.

Simplify validation and graceful scripts, have one template for all cases.

Aviod needless repetition.

Note: There is limit of arguments to be passed to commands, see
https://www.in-ulm.de/~mascheck/various/argmax/, but we are safe for now:

  $ getconf ARG_MAX
  2097152

So we are keeping shell expansion instead of playing with find or other tools.