commit/reset: try to clean up sequencer state

commit/reset: try to clean up sequencer state

When cherry-picking or reverting a sequence of commits and if the final
pick/revert has conflicts and the user uses `git commit` to commit the
conflict resolution and does not run `git cherry-pick --continue` then
the sequencer state is left behind. This can cause problems later. In my
case I cherry-picked a sequence of commits the last one of which I
committed with `git commit` after resolving some conflicts, then a while
later, on a different branch I aborted a revert which rewound my HEAD to
the end of the cherry-pick sequence on the previous branch. Avoid this
potential problem by removing the sequencer state if we're committing or
resetting the final pick in a sequence.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>