libxml2 2.9.3 breaks xml test output

Hello,

I just noticed that last night all built branches failed on my buildfarm 
animal, jaguarundi. They all failed on the "xml" test, and the output is 
essentially the same everywhere:

***************
*** 9,16 ****  LINE 1: INSERT INTO xmltest VALUES (3, '<wrong');                                         ^  DETAIL:
line1: Couldn't find end of Start Tag wrong line 1
 
- <wrong
-       ^  SELECT * FROM xmltest;   id |        data  ----+--------------------
--- 9,14 ----
***************
*** 63,70 ****  LINE 1: SELECT xmlconcat('bad', '<syntax');                                  ^  DETAIL:  line 1:
Couldn'tfind end of Start Tag syntax line 1
 
- <syntax
-        ^  SELECT xmlconcat('<foo/>', NULL, '<?xml version="1.1" 
standalone="no"?><bar/>');    xmlconcat  --------------

The last thing I did in regard to XML on this system was upgrade libxml2 
from 2.9.2 to 2.9.3. I bisected it a a bit and the guilty commit to 
libxml2 is ce0b0d0d8 "Do not print error context when there is none" 
from about two weeks ago.

I have zero experience with libxml2, so no idea if the previous context 
level can be turned on again. IMHO, the libxml2 change is a bug in 
itself; PostgreSQL's error messages are more readable with the XML text 
in them.

-- 
Christian