install.sh: validate $Girocco::Config::chroot a bit more

install.sh: validate $Girocco::Config::chroot a bit more

Make sure that the chroot path does not contain any troublesome
characters that would make it unsafe to use as an alternate
via the GIT_ALTERNATE_OBJECT_DIRECTORIES environment variable.

More recent versions of Git allow a quoting mechanism, but we
can avoid problems simply by prohibiting ':' and ';' in the
chroot path to start with.

Girocco itself does not allow those characters in project
names so it really shouldn't be that much of a burden.

Otherwise at least Git v2.11.1 would be required in order to
properly quote them.  Since chroot already is required to be
an absolute path, it can never start with a double-quote (")
and will therefore never fall afoul of being mistaken for a
quoted alternate with v2.11.1 or later.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>