apache.conf.in: do not allow accelerated access to '_' projects
While the rest of Girocco denies external access to any project
names starting with '_' (which are reserved for internal use),
the accelerated access to packs and loose objects provided by the
web server configuration for non-smart HTTP access provided a
loophole by which some parts of projects starting with '_' could
be accessed.
Close the loophole by only accelerating access for project names
which do NOT start with '_'. Requests for non-smart HTTP access
to loose objects and/or packs for project names starting with '_'
will then be sent to git-http-backend-verify instead which will
send the correct denied result code rather than allowing access.