cherry-pick/revert: add --skip option

git am or rebase have a --skip flag to skip the current commit if the
user wishes to do so. During a cherry-pick or revert a user could
likewise skip a commit, but needs to use 'git reset' (or in the case
of conflicts 'git reset --merge'), followed by 'git (cherry-pick |
revert) --continue' to skip the commit. This is more annoying and
sometimes confusing on the users' part. Add a `--skip` option to make
skipping commits easier for the user and to make the commands more
consistent.

In the next commit, we will change the advice messages hence finishing
the process of teaching revert and cherry-pick "how to skip commits".

Signed-off-by: Rohit Ashiwal <rohit.ashiwal265@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>