ui-shared: pass repo object to print_snapshot_links()

Both call sites of cgit_print_snapshot_links() use the same values for
the snapshot mask and repository name, which are derived from the
cgit_repo structure so let's pass in the structure and access the fields
directly.

Signed-off-by: John Keeping <john@keeping.me.uk>
Reviewed-by: Christian Hesse <mail@eworm.de>