Clean-up of the code path that deals with merge strategy option
handling in "git rebase".
* pw/rebase-cleanup-merge-strategy-option-handling:
rebase: remove a couple of redundant strategy tests
rebase -m: fix serialization of strategy options
rebase -m: cleanup --strategy-option handling
sequencer: use struct strvec to store merge strategy options
rebase: stop reading and writing unnecessary strategy state