cgit.c: respect repo-local 'snapshots' option for --scan-path

The repo-specific 'snapshots' option is bitwise AND'ed with the global
'snapshots' option during parsing, and since the global cgitrc hasn't
been parsed when --scan-path is processed the global 'snapshots' will
always be 0 (i.e. no repo-specific 'snapshots' setting will have any
effect).

This patch fixes the issue by setting the global 'snapshots' mask to
0xFF (hence relying on later parsing of the generated cgitrc repolist
to do the right thing).

Signed-off-by: Lars Hjemli <hjemli@gmail.com>