Make Mushroom Grove/Lit Mushroom Grove behave like forests, move-wise

Prefer the terrain type with the worst movement costs between the base
and the overlay.

This is a forward-port of the following two commits from the 1.12
branch:

 * 18282b542f Fix bug #21971: visual glitch of terrains on 2p - Hornshark Island
 * 35a48f29f9 Make Lit Mushroom Grove stats identical to Mushroom Grove

changelog

@@ -250,6 +250,8 @@ Version 1.13.0-dev:
    * Add [has_ally], [has_enemy] to work around issues reported here: http://forums.wesnoth.org/viewtopic.php?f=21&t=40702
      In a side_filter, [has_ally] and [has_enemy] are corrected / simplified versions of [allied_with] / [enemy_of], this
      is an extension of the bugfixes for [filter_vision].
+   * Mushroom Grove and Lit Mushroom Grove now favor the terrain with the worst
+     movement costs between the base and overlay (bug #21971).
 
 Version 1.11.11:
  * Add-ons server:

data/core/terrain.cfg

@@ -845,7 +845,8 @@ Most units receive 20 to 40% defense in sand."
     name= _ "Mushroom Grove"
     editor_name= _ "Mushroom Grove"
     string=^Uf
-    aliasof=Uft
+    aliasof=_bas,Uft
+    mvt_alias=-,_bas,Uft
     default_base=Uu
     editor_group=cave, forest
 [/terrain_type]
@@ -857,7 +858,8 @@ Most units receive 20 to 40% defense in sand."
     editor_name= _ "Lit Mushroom Grove"
     string=^Ufi
     default_base=Uu
-    aliasof=Uft
+    aliasof=_bas,Uft
+    mvt_alias=-,_bas,Uft
     light=25
     editor_group=cave
 [/terrain_type]

players_changelog

@@ -69,6 +69,9 @@ Version 1.13.0-dev:
    * Fix unit move continuation if enemy discovered (#21372)
    * Support for total conversions, so called "cores". 
    * Partial moves now able to be continued properly in whiteboard (#22231)
+   * Mushroom Grove and Lit Mushroom Grove now favor the terrain with the worst
+     movement costs between the base and overlay (bug #21971).
+
 
 Version 1.11.11:
  * Campaigns: