streamclone: abort when client needs to handle obsmarkers, but doesn't

When client doesn't have any of obsolescence markers exchange capabilities,
then it's safe to say it can't handle obsmarkers. However, if it understands
even one format version, then stream clones are fine -- client can use
"obsmarkers" bundle2 part.

mercurial/bundle2.py

@@ -1700,7 +1700,10 b' def addpartbundlestream2(bundler, repo, '
     includeobsmarkers = False
     if repo.obsstore:
         remoteversions = obsmarkersversion(bundler.capabilities)
-        if repo.obsstore._version in remoteversions:
+        if not remoteversions:
+            raise error.Abort(_('server has obsolescence markers, but client '
+                                'cannot receive them via stream clone'))
+        elif repo.obsstore._version in remoteversions:
             includeobsmarkers = True
 
     filecount, bytecount, it = streamclone.generatev2(repo, includepats,

tests/test-clone-uncompressed.t

@@ -556,6 +556,12 b' Clone non-publishing with obsolescence'
   $ hg debugobsolete -R with-obsolescence
   50382b884f66690b7045cac93a540cba4d4c906f 0 {c17445101a72edac06facd130d14808dfbd5c7c2} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
 
+  $ hg clone -U --stream --config experimental.evolution=0 http://localhost:$HGPORT with-obsolescence-no-evolution
+  streaming all changes
+  remote: abort: server has obsolescence markers, but client cannot receive them via stream clone
+  abort: pull failed on remote
+  [255]
+
   $ killdaemons.py
 
 #endif