Allow -1 as the value of `--backtrace-limit` option

-1 is a legitimate backtrace limit — in fact, it’s the default — so it
should be possible to provide it with the `--backtrace-limit` option.

ruby.c

@@ -1449,7 +1449,7 @@ proc_long_options(ruby_cmdline_options_t *opt, const char *s, long argc, char **
     else if (is_option_with_arg("backtrace-limit", Qfalse, Qtrue)) {
         char *e;
         long n = strtol(s, &e, 10);
-        if (errno == ERANGE || n < 0 || *e) rb_raise(rb_eRuntimeError, "wrong limit for backtrace length");
+        if (errno == ERANGE || n < -1 || *e) rb_raise(rb_eRuntimeError, "wrong limit for backtrace length");
         opt->backtrace_length_limit = (int)n;
     }
     else {

test/ruby/test_rubyoptions.rb

@@ -74,7 +74,7 @@ class TestRubyOptions < Test::Unit::TestCase
   def test_backtrace_limit
     assert_in_out_err(%w(--backtrace-limit), "", [], /missing argument for --backtrace-limit/)
     assert_in_out_err(%w(--backtrace-limit= 1), "", [], /missing argument for --backtrace-limit/)
-    assert_in_out_err(%w(--backtrace-limit=-1), "", [], /wrong limit for backtrace length/)
+    assert_in_out_err(%w(--backtrace-limit=-2), "", [], /wrong limit for backtrace length/)
     code = 'def f(n);n > 0 ? f(n-1) : raise;end;f(5)'
     assert_in_out_err(%w(--backtrace-limit=1), code, [],
                       [/.*unhandled exception\n/, /^\tfrom .*\n/,