clap: Use ArgAction::Count for debug level

clap: Use ArgAction::Count for debug level

Removes a deprecation warning (this is recommended by the clap 3 to 4
migration guide).

    Checking hippotat v1.1.7 (/volatile/rustcargo/Rustup/Hippotat/hippotat)
warning: use of deprecated function `<reporter::LogOpts as clap::Args>::augment_args::parse_from_occurrences`: Replaced with `#[clap(action = ArgAction::Count)]` with a field type of `u8`
  --> src/reporter.rs:18:33
   |
18 |   #[clap(long, short='D', parse(from_occurrences))]
   |                                 ^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(deprecated)]` on by default

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>