rebase -i: remember merge options beyond continue actions

rebase -i: remember merge options beyond continue actions

If the user explicitly specified a merge strategy or strategy
options, continue to use that strategy/option after
"rebase --continue".  Add a test of the corrected behavior.

If --merge is specified or implied by -s or -X, then "strategy and
"strategy_opts" are set to values from which "strategy_args" can be
derived; otherwise they are set to empty strings.  Either way,
their values are propagated from one step of an interactive rebase
to the next via state files.

"do_merge", on the other hand, is *not* propagated to later steps of
an interactive rebase.  Therefore, making the initialization of
"strategy_args" conditional on "do_merge" being set prevents later
steps of an interactive rebase from setting it correctly.

Luckily, we don't need the "do_merge" guard at all.  If the rebase
was started without --merge, then "strategy" and "strategy_opts"
are both the empty string, which results in "strategy_args" also
being set to the empty string, which is just what we want in that
situation.  So remove the "do_merge" guard and derive
"strategy_args" from "strategy" and "strategy_opts" every time.

Reported-by: Diogo de Campos <campos@esss.com.br>
Signed-off-by: Fabian Ruch <bafain@gmail.com>
Helped-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>