Restore cmpVERSION.pl's ability to spot differences in XS files.

Restore cmpVERSION.pl's ability to spot differences in XS files.

Since the refactor to use git tags (instead of a second source tree),
cmpVERSION.pl was only spotting differences in XS files if the corresponding
PM file was also modified. If only the XS file was modified, this was going
undetected.

Remove compare_git_file() - if git has already told us that a file differs,
there's no need to duplicate the work of comparison in Perl.