install.sh: validate $Girocco::Config::reporoot a bit more
In
aace95e56cf84b76 (install.sh: validate $Girocco::Config::chroot
a bit more, 2017-12-12), install.sh started validating
$Girocco::Config::chroot to make sure it could be used safely
in GIT_ALTERNATE_OBJECT_DIRECTORIES without problems.
Correct that and validate $Girocco::Config::reporoot instead
as that was the originally intended check since the chroot
value is not necessarily relevant to the value of
GIT_ALTERNATE_OBJECT_DIRECTORIES at all. D'oh!
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>