actions: introduce function to calculate downgrades

An upgrade operation can also downgrade/remove some format variants. Before this
patch there was no clean way to find out all such variants which will be
removed. This patch adds a function for that.

It will be used in next patch.

Differential Revision: https://phab.mercurial-scm.org/D9618

mercurial/upgrade_utils/actions.py

@@ -428,6 +428,21 b' def find_format_upgrades(repo):'
     return upgrades
 
 
+def find_format_downgrades(repo):
+    """returns a list of format downgrades which will be performed on the repo
+    because of disabled config option for them"""
+
+    downgrades = []
+
+    for fv in allformatvariant:
+        # format variant exist in repo but does not exist in new repository
+        # config
+        if fv.fromrepo(repo) and not fv.fromconfig(repo):
+            downgrades.append(fv)
+
+    return downgrades
+
+
 ALL_OPTIMISATIONS = []