Keep changeset tags when write to file.

[javiersanp]

Currently Josm suport reading of changeset tags in OSM XML files. See this file, for example: ​https://github.com/OSM-es/catastro-import/raw/master/38024/u00210.osm.gz

It includes this change set tags:

<?xml version='1.0' encoding='UTF-8'?>
<osm version='0.6' generator='CatAtom2Osm 1.3.7'>
<changeset>
  <tag k="comment" v="#Spanish_Cadastre_Buildings_Import 38024 Los Llanos de Aridane u00210"/>
  <tag k="generator" v="CatAtom2Osm 1.3.7"/>
  <tag k="url" v="https://wiki.openstreetmap.org/wiki/Spanish_Cadastre/Buildings_Import"/>
  <tag k="source:date" v="2021-09-11"/>
  <tag k="source" v="Direcci&#243;n General del Catastro"/>
  <tag k="type" v="import"/>
</changeset>
<node...

These tags appears correctly in the preferences tab of the upload to OSM dialog.

The problem is that the changeset tags are ommited when writing to a OSM file back again (Ctrl+S).

<?xml version='1.0' encoding='UTF-8'?>
<osm version='0.6' generator='JOSM'>
  <node...

The changeset tags are essential during an import process. There are import processes that requires users to manually review each file in Josm before upload it. With the current changeset tags in the file, the upload is correcty taged and the user don't have to deal with this. Exceptionally, this process could take more time than expected so the user writes the file to disk with his changes to continue editing later. This will ruin the changeset tags and will require that the user write them in the upload dialog.

The propossed solution is to change the code to keep current <changeset> section when writing to a file (.osm or .osm.gz).