sequencer: simplify pick_one_commit()

sequencer: simplify pick_one_commit()

Unless we're rebasing all we do in pick_one_commit() is call
do_pick_commit() and return its result. Simplify the code by returing
early if we're not rebasing so that we don't have to continually call
is_rebase_i() in the rest of the function. Note that there are a couple
of conditions that do not call is_rebase_i() but they check for either
an "edit" or a "fixup" command, both of which imply we're rebasing.

As the conditional blocks are all mutually exclusive (either the
conditions are mutually exclusive, or an earlier conditional block
that would match a later one contains a "return" statement) chain
them together with "else if" to make that clear.

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