Simplify the way of appending comma to stringInfo

Hi Hackers,

In a lot places, there are logic of appending comma separators in a pattern like:

```

for (int i = 0; i < len; i ++)

{

    if (i > 0)

       appendStringInfoString(", ");

    appendStringInfo(some-item);

}

```

This pattern uses an "if" check and two appendStringInfoString() to build a comma-delimited string. 

This can be simplified as:

```

const char *sep = "";

for (int i = 0; i < len; i ++)

{

     appendStringInfo("%s%s", sep, some-item);

     sep = ", ";

}

```

The new pattern avoids the "if" check, and combines two appendStringInfoString() into a single appendStringInfo(). I think the new pattern is neater and faster.

The old patterns are used in a lot of places, and there are some usages of the new pattern as well. Instead of creating a big cleanup patch, I just applied the new pattern to a single file for now to see if the hacker group likes this change.

Best regards,

==