#listmerge: Fix post-sort with templates

[Derugon]

Motivation

The #listfilter, #listmap, #listunique, and #listmerge functions. take a list, and return a new list, doing a specific operation using a pattern or template.

Since they operate in the list from left to right, a sortmode parameter can be provided to sort the list before (sortmode = presort), after (sortmode = postsort), or both before and after (sortmode = pre/postsort) the operation is applied.

However, when given a template, the #listmerge function does not try to sort values after having applied its template, so the postsort option has no effect and pre/postsort works as presort. This is not the case when a pattern is provided instead, see the associated test case.

Proposed changes

Add a post-sorting step to #listmerge when templates are used for matching/merging values.