executor: fix group_concat when chunk size is set to 1

[XuHuaiyu]

What have you changed? (mandatory)

Before this PR, the following sql would return wrong result.

create table idx_agg (a int, b int, index (b));
insert idx_agg values (1, 1), (1, 2), (2, 2);
set @@tidb_max_chunk_size=1;
tidb> select group_concat(b) from idx_agg group by b;
+-----------------+
| group_concat(b) |
+-----------------+
| 1              |
| 22            |
+-----------------+

The PR fix the problem:

tidb> select group_concat(b) from idx_agg group by b;
+-----------------+
| group_concat(b) |
+-----------------+
| 1             |
| 2,2         |
+-----------------+
2 rows in set (0.00 sec)

How does this bug occur？

This bug is caused by this line.
When max_chunk_size is set to 1, rowInGroup pass into the function
contains only 1 row every time. As we can see, separator be appended
here would be truncated out of the loop.

How does this bug be fixed?

This commit first check whether p.buffer != nil, which means we've
got part of the result for a group, if it's true, we'll append separator at
the end of p.buffer. Through this, we can avoid truncating separator
wrongly. Then we append the string be consist of the values. If
this string is null, we'll truncated the separator which is appended at
the first.

What is the type of the changes? (mandatory)

Bug-fix

How has this PR been tested? (mandatory)

unit test

Does this PR affect documentation (docs/docs-cn) update? (mandatory)

Does this PR affect tidb-ansible update? (mandatory)

no

Does this PR need to be added to the release notes? (mandatory)

no

Refer to a related PR or issue link (optional)

Benchmark result if necessary (optional)

Add a few positive/negative examples (optional)

[winoros]

@XuHuaiyu The 800+ line change is all to fix the problem described in title?

[XuHuaiyu]

@winoros Noop, most of the lines are from #7281 .
It would be nice if you can review it first.

The following functions fix and test the bug.
func_group_concat.go: func(e *groupConcat) UpdatePartialResult
func_avg_test.go: TestMergePartialResult4GroupConcat

[zz-jason]

@XuHuaiyu can you remove the irrelevant changes so that we can be focused on the bugfix itself and merge this PR as soon as possible?

[XuHuaiyu]

PTAL @zz-jason @winoros

[winoros]

lgtm

[XuHuaiyu]

PTAL @zz-jason

[zz-jason]

lgtm

[XuHuaiyu]

/run-all-tests