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 returning
early if we're not rebasing so that we don't have to repeatedly 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.

The only block that does not return early is the one guarded by
"!res". Move the return into that block to make it clear that after
recording the commit as rewritten, all we do is return from the
function.

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.

While we could remove "res" from the conditions below "if (!res)"
they are left alone because, when we start using an enum in the next
commit, it makes it clear that these clauses are handling cases where
there are conflicts.

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