t7810: avoid unportable use of "echo"

t7810: avoid unportable use of "echo"

Michael J Gruber noticed that under /bin/dash this test failed
(as is expected -- \n in the string can be interpreted by the
command), while it passed with bash.  We probably could work it
around by using backquote in front of it, but it is safer and
more readable to avoid "echo" altogether in a case like this.

Signed-off-by: Junio C Hamano <gitster@pobox.com>