upgrade: move optimisation to something more declarative

This is not great yet, but still better than the previous state. This get use
one step closer to having all the possible "actions" clearly declared and moved
in a dedicated module.

Differential Revision: https://phab.mercurial-scm.org/D9475

mercurial/upgrade.py

@@ -541,14 +541,15 b' legacy_opts_map = {'
     b'redeltafulladd': b're-delta-fulladd',
 }
 
+ALL_OPTIMISATIONS = []
 
-def findoptimizations(repo):
-    """Determine optimisation that could be used during upgrade"""
-    # These are unconditionally added. There is logic later that figures out
-    # which ones to apply.
-    optimizations = []
 
-    optimizations.append(
+def register_optimization(obj):
+    ALL_OPTIMISATIONS.append(obj)
+    return obj
+
+
+register_optimization(
         improvement(
             name=b're-delta-parent',
             type=OPTIMISATION,
@@ -568,7 +569,7 b' def findoptimizations(repo):'
         )
     )
 
-    optimizations.append(
+register_optimization(
         improvement(
             name=b're-delta-multibase',
             type=OPTIMISATION,
@@ -592,7 +593,7 b' def findoptimizations(repo):'
         )
     )
 
-    optimizations.append(
+register_optimization(
         improvement(
             name=b're-delta-all',
             type=OPTIMISATION,
@@ -610,7 +611,7 b' def findoptimizations(repo):'
         )
     )
 
-    optimizations.append(
+register_optimization(
         improvement(
             name=b're-delta-fulladd',
             type=OPTIMISATION,
@@ -630,7 +631,12 b' def findoptimizations(repo):'
         )
     )
 
-    return optimizations
+
+def findoptimizations(repo):
+    """Determine optimisation that could be used during upgrade"""
+    # These are unconditionally added. There is logic later that figures out
+    # which ones to apply.
+    return list(ALL_OPTIMISATIONS)
 
 
 def determineactions(repo, deficiencies, sourcereqs, destreqs):