sequencer: update `done_nr' when skipping commands in a todo list

sequencer: update `done_nr' when skipping commands in a todo list

In a todo list, `done_nr' is the number of commands that were executed
or skipped, but skip_unnecessary_picks() did not update it.

This variable is mostly used by command prompts (ie. git-prompt.sh and
the like).  As in the previous commit, this inconsistent behaviour is
not a problem yet, but it would start to matter at the end of this
series the same reason.

Signed-off-by: Alban Gruin <redacted>
Signed-off-by: Junio C Hamano <redacted>