changegroup: simplify logic around enabling changegroup 03

There was multiple spot that took care of adding '03' as supported changegroup
version for different condition. We gather them all in one location for
simplicity.

The 'supportedincomingversions' function is now doing nothing, but I kept it
around because it looks like a great hooking point for extension.

(Note that we should probably just get changegroup3 out of experimental now, But
that would be a patch with a much wider scope).

mercurial/changegroup.py

@@ -877,16 +877,14 b' class cg3packer(cg2packer):'
 def allsupportedversions(repo):
     versions = set(_packermap.keys())
     if not (repo.ui.configbool('experimental', 'changegroup3') or
-            repo.ui.configbool('experimental', 'treemanifest')):
+            repo.ui.configbool('experimental', 'treemanifest') or
+            'treemanifest' in repo.requirements):
         versions.discard('03')
     return versions
 
 # Changegroup versions that can be applied to the repo
 def supportedincomingversions(repo):
-    versions = allsupportedversions(repo)
-    if 'treemanifest' in repo.requirements:
-        versions.add('03')
-    return versions
+    return allsupportedversions(repo)
 
 # Changegroup versions that can be created from the repo
 def supportedoutgoingversions(repo):
@@ -899,7 +897,6 b' def supportedoutgoingversions(repo):'
         # support versions 01 and 02.
         versions.discard('01')
         versions.discard('02')
-        versions.add('03')
     return versions
 
 def safeversion(repo):