t7800 test failure

t7800 fails on systems where readlink (GNUism?) is not available.
An easy workaround for the very basic readlink usage of this test
would be to use a shell function like this:

---

diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index ff7a9e9..be3d19f 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh

@@ -420,6 +420,7 @@ run_dir_diff_test 'difftool --dir-diff when

worktree file is missing' '
 '

 write_script .git/CHECK_SYMLINKS <<\EOF
+readlink() { ls -ld "$1" | sed 's/.* -> //'; }
 for f in file file2 sub/sub
 do
        echo "$f"