builtin/rebase: always store allocated string in options.strategy

The `struct rebase_options::strategy` field is a `char *`, but we do end
up assigning string constants to it in two cases:

  - When being passed a `--strategy=` option via the command line.

  - When being passed a strategy option via `--strategy-option=`, but
    not a strategy.

This will cause warnings once we enable `-Wwrite-strings`.

Ideally, we'd just convert the field to be a `const char *`. But we also
assign to this field via the GIT_TEST_MERGE_ALGORITHM envvar, which we
have to strdup(3P) into it.

Instead, refactor the code to make sure that we only ever assign
allocated strings to this field.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

builtin/rebase.c

@@ -1061,6 +1061,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 {
 	struct rebase_options options = REBASE_OPTIONS_INIT;
 	const char *branch_name;
+	const char *strategy_opt = NULL;
 	int ret, flags, total_argc, in_progress = 0;
 	int keep_base = 0;
 	int ok_to_skip_pre_rebase = 0;
@@ -1175,7 +1176,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			PARSE_OPT_OPTARG, parse_opt_rebase_merges),
 		OPT_BOOL(0, "fork-point", &options.fork_point,
 			 N_("use 'merge-base --fork-point' to refine upstream")),
-		OPT_STRING('s', "strategy", &options.strategy,
+		OPT_STRING('s', "strategy", &strategy_opt,
 			   N_("strategy"), N_("use the given merge strategy")),
 		OPT_STRING_LIST('X', "strategy-option", &options.strategy_opts,
 				N_("option"),
@@ -1484,13 +1485,12 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		}
 	}
 
-	if (options.strategy_opts.nr && !options.strategy)
-		options.strategy = "ort";
-
-	if (options.strategy) {
-		options.strategy = xstrdup(options.strategy);
+	if (strategy_opt)
+		options.strategy = xstrdup(strategy_opt);
+	else if (options.strategy_opts.nr && !options.strategy)
+		options.strategy = xstrdup("ort");
+	if (options.strategy)
 		imply_merge(&options, "--strategy");
-	}
 
 	if (options.root && !options.onto_name)
 		imply_merge(&options, "--root without --onto");