Fixed #17375 -- Changed makemessages to use xgettext with --files-from

Changed the way makemessages invokes xgettext from one call per
translatable file to one call per locale directory (using --files-from).
This allows to avoid https://savannah.gnu.org/bugs/index.php?35027 and,
as a positive side effect, speeds up localization build.

tests/i18n/commands/templates/test.html

@@ -80,3 +80,9 @@ continued here.{% endcomment %}
   should be trimmed.
 {% endblocktrans %}
 {% trans "I'm on line 82" %}
+
+{% blocktrans trimmed count counter=mylist|length %}
+First `trans`, then `blocktrans` with a plural
+{% plural %}
+Plural for a `trans` and `blocktrans` collision case
+{% endblocktrans %}