test: make test-extdiff resilient to */gnubin/echo...
test: make test-extdiff resilient to */gnubin/echo
My Mac test machine has 'echo' in '/opt/local/libexec/gnubin/echo' since, well,
GNU is not BSD.
Also, I feel it need to be said about using regexes:
Some people, when confronted with a problem, think "I know, I'll use regular
expressions." Now they have two problems.
