bup-cron: create the missing directories more carefully
bup-cron: create the missing directories more carefully

chdir() changes the current working directory, but only when the directory
already exist. make_dirs_helper() avoid that side-effect and it simplify
a little bit the code logic.

Signed-off-by: Patrick Rouleau <63d5de1631bda7ef519bef8dad039f592a2f1542@gmail.com>

blueyed/bup-cron,anarcat/bup-cron,blueyed/bup-cron,anarcat/bup-cron