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,
==
Chao Li (Evan)
---------------------
HighGo Software Co., Ltd.
https://www.highgo.com/