Merge branch 'ak/t7800-wo-readlink' into maint

One among four invocations of readlink(1) in our test suite has
been rewritten so that the test can run on systems without the
command (others are in valgrind test framework and t9802).

* ak/t7800-wo-readlink:
t7800: readlink may not be available

diff --cc t/t7800-difftool.sh
Simple merge