bundle2: detect bundle2 stream/request on /HG2./ instead of /HG2Y/

To support more bundle2 formats, we need a wider detection of bundle2-family
streams. The various places what were explicitly detecting the full magic string
are now matching on the first three characters of it.

mercurial/exchange.py

@@ -32,7 +32,7 b' def readbundle(ui, fh, fname, vfs=None):'
         if alg is None:
             alg = changegroup.readexactly(fh, 2)
         return changegroup.cg1unpacker(fh, alg)
-    elif version == '2Y':
+    elif version.startswith('2'):
         return bundle2.getunbundler(ui, fh, header=magic + version)
     else:
         raise util.Abort(_('%s: unknown bundle version %s') % (fname, version))
@@ -1168,7 +1168,10 b' def getbundle(repo, source, heads=None, '
     when the API of bundle is refined.
     """
     # bundle10 case
-    if bundlecaps is None or 'HG2Y' not in bundlecaps:
+    usebundle2 = False
+    if bundlecaps is not None:
+        usebundle2 = util.any((cap.startswith('HG2') for cap in bundlecaps))
+    if not usebundle2:
         if bundlecaps and not kwargs.get('cg', True):
             raise ValueError(_('request for bundle10 must include changegroup'))
 

mercurial/wireproto.py

@@ -363,7 +363,9 b' class wirepeer(peer.peerrepository):'
             opts[key] = value
         f = self._callcompressable("getbundle", **opts)
         bundlecaps = kwargs.get('bundlecaps')
-        if bundlecaps is not None and 'HG2Y' in bundlecaps:
+        if bundlecaps is None:
+            bundlecaps = () # kwargs could have it to None
+        if util.any((cap.startswith('HG2') for cap in bundlecaps)):
             return bundle2.getunbundler(self.ui, f)
         else:
             return changegroupmod.cg1unpacker(f, 'UN')