branch: allow recursion with no tracking name

Creating a branch across submodules from a commit that no ref points
  at fails, with the helper's usage text reprinted as an error:

    submodule 'sub': usage: git submodule--helper create-branch [...]
    fatal: submodule 'sub': cannot create branch 'branch-a'

`submodule_create_branch()` runs the helper in a child process because
  `install_branch_config_multiple_remotes()` cannot write config into a
  submodule, and passes the branch name, the start oid and the tracking
  name as three positionals.
`dwim_branch_start()` leaves the tracking name NULL where the start
  point named no ref, and `strvec_pushl()` stops at the first NULL, so
  the child receives two positionals.
`module_create_branch()` requires exactly three and prints its usage.

Make the third positional optional, since a start point that named no
  ref has no tracking name to give and the recursion has nothing to
  track in the submodule either.
Push it separately in the caller too: relying on `strvec_pushl()` to
  stop early leaves the argument dropped by accident rather than by
  intent, and a reader has to know where the terminator falls to see
  that it can go missing at all.

961b130d20 (branch: add --recurse-submodules option for branch
  creation, 2022-01-28) introduced both sides.

This is the same NULL tracking name as the previous patch, reached one
  step earlier: the dry-run pass over the submodules runs before the
  superproject's own `setup_tracking()` call, so with a submodule
  present this failure hides the abort that patch removes.
The new test therefore needs that patch under it.

Assisted-by: An LLM.
Signed-off-by: Volodymyr Vriukalo <0@zitro.id>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -760,7 +760,15 @@ static int submodule_create_branch(struct repository *r,
 		break;
 	}
 
-	strvec_pushl(&child.args, name, start_oid, tracking_name, NULL);
+	/*
+	 * The tracking name is absent when the start point named no ref.
+	 * Push it separately: strvec_pushl() stops at the first NULL, so
+	 * passing it inline would drop the argument by accident rather
+	 * than by intent.
+	 */
+	strvec_pushl(&child.args, name, start_oid, NULL);
+	if (tracking_name)
+		strvec_push(&child.args, tracking_name);
 
 	if ((ret = start_command(&child)))
 		return ret;

builtin/submodule--helper.c

@@ -3336,7 +3336,7 @@ static int module_create_branch(int argc, const char **argv, const char *prefix,
 		OPT_END()
 	};
 	const char *const usage[] = {
-		N_("git submodule--helper create-branch [-f|--force] [--create-reflog] [-q|--quiet] [-t|--track] [-n|--dry-run] <name> <start-oid> <start-name>"),
+		N_("git submodule--helper create-branch [-f|--force] [--create-reflog] [-q|--quiet] [-t|--track] [-n|--dry-run] <name> <start-oid> [<start-name>]"),
 		NULL
 	};
 	struct repo_config_values *cfg = repo_config_values(the_repository);
@@ -3345,13 +3345,14 @@ static int module_create_branch(int argc, const char **argv, const char *prefix,
 	track = cfg->branch_track;
 	argc = parse_options(argc, argv, prefix, options, usage, 0);
 
-	if (argc != 3)
+	if (argc < 2 || argc > 3)
 		usage_with_options(usage, options);
 
 	if (!quiet && !dry_run)
 		printf_ln(_("creating branch '%s'"), argv[0]);
 
-	create_branches_recursively(the_repository, argv[0], argv[1], argv[2],
+	create_branches_recursively(the_repository, argv[0], argv[1],
+				    argc > 2 ? argv[2] : NULL,
 				    force, reflog, quiet, track, dry_run);
 	return 0;
 }

t/t3207-branch-submodule.sh

@@ -115,6 +115,19 @@ test_expect_success 'should move a branch to a start point that names no ref' '
 	)
 '
 
+test_expect_success 'should recurse into submodules from a start point that names no ref' '
+	test_when_finished "reset_test" &&
+	(
+		cd super &&
+		oid=$(git rev-parse HEAD) &&
+		git branch --recurse-submodules branch-a "$oid" &&
+		git rev-parse branch-a &&
+		git -C sub rev-parse branch-a &&
+		git -C sub/sub-sub rev-parse branch-a &&
+		git -C second/sub rev-parse branch-a
+	)
+'
+
 test_expect_success 'should ignore submodule.recurse when not creating branches' '
 	test_when_finished "reset_test" &&
 	(