tests: avoid nonportable {foo,bar} glob

tests: avoid nonportable {foo,bar} glob

Unlike bash and ksh, dash and busybox ash do not support brace
expansion (as in 'echo {hello,world}').  So when dash is sh,
t6009.13 (set up dodecapus) ends up pass a string beginning with
"root{1,2," to "git merge" verbatim and the test fails.

Fix it by introducing a variable to hold the list of parents for
the dodecapus and populating it in a more low-tech way.

While at it, simplify a little by combining this setup code with the
test it sets up for.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>