show --continue/skip etc. consistently in synopsis

show --continue/skip etc. consistently in synopsis

Command mode options that the user can choose one among many are
listed like this in the documentation:

    git am (--continue | --skip | --abort | --quit)

They are listed on a single line and in parenthesis, because they
are not optional.

But documentation pages for some commands deviate from this norm.
Fix the merge and rebase docs to match this style.

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