xml.c passed format = 1 to xmlNodeDump(), resulting in sometimes getting
extra whitespace (newlines + spaces) in the output.  We don't really want
that, first because whitespace might be semantically significant in some
XML uses, and second because it happens only very inconsistently.  Only
one case in our regression tests is affected.

This potentially affects the results of xpath() and the XMLTABLE construct,
when emitting nodeset values.

Note that the older code in contrib/xml2 doesn't do this; it seems
to have been an aboriginal bad decision in commit ea3b212f.

While this definitely seems like a bug to me, the small number of
complaints to date argues against back-patching a behavioral change.
Hence, fix in HEAD only, at least for now.

Per report from Jean-Marc Voillequin.

Discussion: https://postgr.es/m/1EC8157EB499BF459A516ADCF135ADCE3A23A9CA@LON-WGMSX712.ad.moodys.net