sequencer: use struct strvec to store merge strategy options

sequencer: use struct strvec to store merge strategy options

The sequencer stores the merge strategy options in an array of strings
which allocated with ALLOC_GROW(). Using "struct strvec" avoids manually
managing the memory of that array and simplifies the code.

Aside from memory allocation the changes to the sequencer are largely
mechanical, changing xopts_nr to xopts.nr and xopts[i] to xopts.v[i]. A
new option parsing macro OPT_STRVEC() is also added to collect the
strategy options.  Hopefully this can be used to simplify the code in
builtin/merge.c in the future.

Note that there is a change of behavior to "git cherry-pick" and "git
revert" as passing “--no-strategy-option” will now clear any previous
strategy options whereas before this change it did nothing.

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