uucore/num_format: properly display 10ᵖ where p is the precision

`seq --format %.2g 10 10` would display `1` because the precision would
not allow room for the decimal point, and the `0` in `10` would be
trimmed as an insignificant trailing `0`.

This has been fixed by only trimming trailing `0` in the presence of a
decimal point.

tests/by-util/test_seq.rs

@@ -766,3 +766,23 @@ fn test_invalid_zero_increment_value() {
         .no_stdout()
         .usage_error("invalid Zero increment value: '0'");
 }
+
+#[test]
+fn test_power_of_ten_display() {
+    new_ucmd!()
+        .args(&["-f", "%.2g", "10", "10"])
+        .succeeds()
+        .stdout_only("10\n");
+}
+
+#[test]
+fn test_default_g_precision() {
+    new_ucmd!()
+        .args(&["-f", "%010g", "1e5", "1e5"])
+        .succeeds()
+        .stdout_only("0000100000\n");
+    new_ucmd!()
+        .args(&["-f", "%010g", "1e6", "1e6"])
+        .succeeds()
+        .stdout_only("000001e+06\n");
+}