Fix vfprintf and derived functions badly handling +/` flag conflict (#1269)

Author: GabrielRavier

libc/stdio/fmt.c

@@ -76,9 +76,9 @@
 #define FLAGS_PRECISION 0x20
 #define FLAGS_ISSIGNED  0x40
 #define FLAGS_NOQUOTE   0x80
+#define FLAGS_REPR      0x100
 #define FLAGS_QUOTE     FLAGS_SPACE
 #define FLAGS_GROUPING  FLAGS_NOQUOTE
-#define FLAGS_REPR      FLAGS_PLUS
 
 #define __FMT_PUT(C)              \
   do {                            \

test/libc/stdio/snprintf_test.c

@@ -27,3 +27,11 @@ TEST(snprintf, testVeryLargePrecision) {
   ASSERT_EQ(i, 9999);
   ASSERT_EQ(strlen(buf), 511);
 }
+
+TEST(snprintf, testPlusFlagOnChar) {
+  char buf[10] = {};
+  int i = snprintf(buf, sizeof(buf), "%+c", '=');
+
+  ASSERT_EQ(i, 1);
+  ASSERT_STREQ(buf, "=");
+}