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>

parse-options-cb.c

@@ -208,6 +208,22 @@ int parse_opt_string_list(const struct option *opt, const char *arg, int unset)
 	return 0;
 }
 
+int parse_opt_strvec(const struct option *opt, const char *arg, int unset)
+{
+	struct strvec *v = opt->value;
+
+	if (unset) {
+		strvec_clear(v);
+		return 0;
+	}
+
+	if (!arg)
+		return -1;
+
+	strvec_push(v, arg);
+	return 0;
+}
+
 int parse_opt_noop_cb(const struct option *opt, const char *arg, int unset)
 {
 	return 0;