Avoid re-writing files unnecessarily in src/tools/copyright.pl.

Avoid re-writing files unnecessarily in src/tools/copyright.pl.

The existing coding resulted in touching every copyright-containing
file in the tree, even if it was already up to date.  That doesn't
matter much for the annual run, but it's an annoyance if you try
to use the script for mop-up at the close of a devel cycle, as
I just did.

Discussion: https://postgr.es/m/266030.1649685473@sss.pgh.pa.us