Keep the sign of zero when negating a number literal (#3662)

Since jq 1.8.0, negating a number literal computed 0 - x, so -0 and
0 | -. printed 0 while (0+0) | -. printed -0.

Flip the sign bit with decNumberCopyNegate() instead, so -0 prints -0
again as it did in jq 1.7, and -(-0) is still 0.

src/jv.c

@@ -758,7 +758,7 @@ jv jv_number_negate(jv n) {
   if (JVP_HAS_FLAGS(n, JVP_FLAGS_NUMBER_LITERAL)) {
     jvp_literal_number* m = jvp_literal_number_alloc(jvp_dec_number_ptr(n)->digits);
 
-    decNumberMinus(&m->num_decimal, jvp_dec_number_ptr(n), DEC_CONTEXT());
+    decNumberCopyNegate(&m->num_decimal, jvp_dec_number_ptr(n));
     jv r = {JVP_FLAGS_NUMBER_LITERAL, 0, 0, 0, {&m->refcnt}};
     return r;
   }

tests/jq.test

@@ -2443,6 +2443,11 @@ true
 null
 true
 
+# Unary negation keeps the sign of zero
+[-0, -(-0), (.[] | -.)] | map(tojson)
+[0, -0]
+["-0","0","-0","0"]
+
 . |= try . catch .
 1
 1