skip testing .github/README.md if doesn't appear to be a README

git on windows at least of 2.37.1 does not checkout symbolic links
as symbolic links, leaving them as files containing the filename
linked to.

This meant this test against .github/README.md failed, since it only
contained "../README": nothing that appears to be a copyright message

I considered testing that .github/README.md was a symbolic link
but 8975221 suggests that it may eventually become more distinct
from the root README file, so instead check its size.

Author: tonycoz

t/porting/copyright.t

@@ -31,7 +31,9 @@ my ($opt) = @ARGV;
 my $readme_year = readme_year();
 my $v_year = v_year();
 my $gh_readme_year;
-if (-e "../.github/README.md")
+# git on windows renders symbolic links as a file containing
+# the file linked to
+if (-e "../.github/README.md" && -s "../.github/README.md" > 80)
 {
   $gh_readme_year = readme_year(".github/README.md");
 }