This changes the way revids not present in the array are handled:

- For gaps in the array, the next present revid is used.

- For revids beyond the array, the new STEP_FUTURE is used instead of
  the last revid in the array.

In both cases, we'll get debug logging of what's going on.

v2: Rename stepping->step
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/756fe3d75b1e91ef812fc1fd3f70337e9c571d91.1616764798.git.jani.nikula@intel.com